[XtremeRevolution]

How to Change Gear Oil/Differential Fluid​

Overview:
This tutorial will walk you through the process of changing the gear oil/differential fluid on the 124 Spider.

A note regarding fluid selection:
According to Ameridian's blog, FCA recommends a 75W-90 GL-5 hypoid gear oil with FCA's multi-vehicle limited slip additive (for the Abarth). I called AMSOIL's technical team and they explained that our 75W-90 Severe Gear actually has limited slip friction modifiers already formulated in the gear oil, and that I don't need to add any extra. I already had some additive on hand, but decided not to use it. If you end up using something else, ensure that it already has limited slip friction modifiers formulated in the oil, or plan to purchase FCA's additive. If you have a Classica/Lusso, you don't have a limited slip differential, so you don't have to worry about the limited slip additive. Use whichever fluid you feel comfortable with; this thread is intended only to provide instruction on how to change the fluid.

I used AMSOIL's Sever Gear 75W-90 because it has exceptional extreme pressure protection, it runs cooler than most fluids, is already formulated for limited slip differentials, and is now available (as of August 1st 2018) in a package that makes filling easier than anything else on the market. You can find it here:

https://www.amsoil.com/shop/by-product/gear-lube/severe-gear-75w-90/?code=SVGPK-EA&zo=5224266

A note regarding service intervals:
I like to change gear oil often based on the ISO particle count oil analysis reports I've seen of gear oils and the effects silicon have on metallic wear. Silicon is a contaminant that enters the differential through the breather. This wear is typically out of range of a typical oil analysis report (5-15 microns) and will not show up on cheaper reports until it's too late. I like to change the fluid to get the abrasive silcon out before it causes wear.

Tools Required:
- 23mm and 24mm sockets and ratchet
- Shop towels
- Torque wrench
- Ramps/jack/jackstands
- Oil pan

Part Required:
- 1 Quart of 75W-90 Hypoid Gear Oil (see comment above). Actual fill spec is 0.63 quart/0.6 liter.
- Optional: Replacement washers for the drain/fill plugs (Available on Amazon.com), part number 995-41-400 according to a thread on Miata.net. I generally replace these every other service or if I see one is damaged.

Procedure:
1. Lift the car securely and as level as possible. The differential is on the back. Note the location of the drain (blue) and fill (red) plugs.

2. Remove the fill plug first, and remove the drain plug only if the fill plug has been removed.

3. Clean the metal shavings off of the drain plug magnet. Reinstall the drain plug and torque to ~40N-m (official spec is 40-53 N-m).

4. If you're using the AMSIOL Easy-Pack, remove the nozzle, peel back the seal, cut the tip off of the nozzle, and screw the nozzle back on.

5. Fill the differential until fluid starts coming out. Wait for the fluid to stop dripping, and thread the fill plug back on. Note, keep your oil pan underneath while you're filling. I made a video of this one to show how easy it is.

6. Re-tighten the fill plug and torque to ~40N-m (official spec is 40-53 N-m).

Here's what my factory differential oil looked like at ~4700 miles.

 

[XtremeRevolution]

Is this Abarth specific?

I don't think so. The non-abarth should have the same drain and fill plugs in the same location. I know for a fact the fill spec is the same.

Sent from my BBF100-2 using Tapatalk

 

[XtremeRevolution]

@XtremeRevolution

Slightly off topic, I know your car is a manual but would you happen to know what type of ATF I should use when changing out the auto trans fluid? The manual states the MOPAR brand but obviously I'd like to see if there is a more accessible/better off the shelf alternative.

Thanks.

I'm not sure about that to be honest with you. It looks like the fluid needs to meet JWS 3309 spec, which I don't think you'll find off the shelf. Our multi-vehicle OE and Signature Series ATFs meet that specification, if you're interested.

 

[XtremeRevolution]

Sure, I'd be interested. I guess what I mean by off the shelf is anything on Amazon or I could buy online that's not OEM/dealership only.

It looks like Mobil 1 has a 3309 ATF also: https://amzn.to/2MaDBAb

Shoot me a PM with what you'd be interested in ordering and I can get you a quote shipped.

 

[XtremeRevolution]

Are the replacement washers the same dimensions as the ones that go on the manual transmission fill and drain bolts? I still have a few of those kicking around.

I think so but don't quote me on that.

 

[XtremeRevolution]

Having already changed the diff (and fluid) in my Classica, I can verify there's no difference from what you have.

Edit: as a side note, I installed a complete "pumpkin" from an Abarth, and used Mopar 75W85 GL5 synthetic w/ LSD additive. It's been doing fine since early this spring with both race and street use. I'll examine the fluid closely this fall.

By "examine" do you mean send it out for oil analysis?

How hot does your differential get?

 

[XtremeRevolution]

Xtreme, when you say you like to change the fluid often, how often is that? I am just at 2900 miles. I am not sure what all the issues with shifting and such. Best I can tell, Spidy shifts just fine, and any shifting issues are definitely operator induced

With regard to service intervals, I was talking about differentials alone. The issue here is the constant heating and cooling, while the breather is exposed to road dust on a regular basis, causing silicon contamination that is abrasive. This, combined with a low sump volume, causes wear over time. This isn't as much an issue for transmissions as it is for differentials, which are closer to the road and lower to the ground. I will most likely change my differential oil every 2 years with my driving. I drive around 3000 miles a year.

As for when to change the MTF, you'll start to notice shifting quality degrade over time, but generally I recommend every 30,000-45,000 miles. Changing it early won't hurt, and getting the break-in oil out early on the initial change will certainly help as well.

Keep in mind, as a bit of a perfectionist, I'll recommend people do what's best for their vehicles, not the minimum necessary.

 

[XtremeRevolution]

Extreme
changed my MTransmission Oil today, with the pump it took 2 1/3 QUARTS. Mechanic asked me if there was a OIL gage for the trans, I told him mostly likely NO. Anyway when he drained it the bold had metal on it. Once finished screwed the draining bolt and started to fill from the above bolt till it started to overflow.

Is 2 1/3 quarts what's needed. tia

Yes that's approximately correct. The method you described is the way it should be done.

 

[XtremeRevolution]

1) no, but that's not a bad idea, come to think of it.

2) I haven't measured it. What kind of use do you think would produce the highest peak number or a consistently high number, for sake of measuring? I have thermocouples and thermally conductive grease, so I can put a probe just about anywhere on the diff. I'm guessing somewhere out of airflow would be good, but not quite sure where the best place to be to get closest to the "inside view."

Back of the diff, lower half. That would be about the best place you could put it. As for driving conditions, high load, low speed, with low airflow and little cool-down time. Auto-x racing would be a good example.

Let me know if you want to get oil analysis done and I can get you a kit with a good ASTM Certified lab.

 

[XtremeRevolution]

I've recently learned that the recommended differential gear oil should be 75W-90 SG1, rather than 75W-90 GL5 that my previous research had revealed. That seems to be what you have used, if SG1 and Amsoil Severe Gear are synonymous. Do your data sheets explain the difference between the two?

Another question - the recommended ATF was JWS3309, but now seems to be JWS3324. Is there much of a difference? In identifying JWS3324 products, it appears that Dexron VI ATF products seem to be the equivalent.

"SG1" is just a placeholder for a fluid specification that Mazda does not release to the public. Similar to transmission fluid; Mazda simply won't release their spec, likely in order to sell more of their overpriced fluid, not so much because a synchromesh or other GL-4 manual transmission fluid won't work. In this case, "SG1" is meaningless, while GL-5 is an API rating for extreme pressure additive concentration.

Good question Dan. I switched rear diff fluid to the Amsoil Severe Gear Oil about 3,000 miles ago. The original advice was that Amsoil 75W-90 did not need a friction modifier but I'm wondering if that is correct. My last couple of times on my local track we ran the course clockwise which results in us turning in tight to a downhill chicane. The inside rear wheel gets unweighted as you turn into the chicane and start downhill and spins for at least 25 feet if you don't lift on the throttle some. I didn't pay any attention to it innitially but now I'm thinking the limited slip should actually shift the drive wheel to the outside wheel. Am I wrong in this thinking? If not, why didn't it do it? I'm not hearing any noises or experiencing any problems with the rear differential so I assume everything is as it should be.

AMSOIL 75W-90 Severe Gear doesn't need an additional friction modifier, because it already has a friction modifier built into the oil. It is already formulated for use in applications with limited slip differentials. I verified this with AMSOIL's technical services team. The limited slip additive is only used when what's in the gear oil is not enough, and I was told that is essentially limited to large differentials utilizing a clutch-based limited slip additive. In this case, it appears that the additive allows more slip, so the clutches don't chatter as they attempt to slip. Adding more additive, it would seem, would have the opposite effect of what you might be looking for if you suspect the differential is slipping too much.

Keep in mind, limited slip just means that; there is a limited amount of slip allowed for a given wheel. I recall GWR also noting that this car's LSD is a bit slow to respond, which is why they upgraded. A limited lip differential doesn't transfer power from one wheel to the other like an active AWD system might; it simply uses friction inside the differential, created by a speed differential between the two wheels, to generate force on the opposing wheel when one wheel loses traction. I'm not sure just how much that one wheel needs to be spinning before the other begins to produce significant power, as the differential allows enough slip to take tight turns at lower speeds without chatter or the outer wheel skipping. In your case, the inner wheel will still spin more than the outer wheel will, while the LSD is working, but because the outer wheel is still spinning, you are likely within the allowable amount of slip. If your outer wheel wheel were to stop spinning entirely, the LSD would begin transferring more power to it, while the outer wheel still slips. An LSD doesn't guarantee both wheels are spinning at the same speed.

I was directing the question at Andrei Pop (@XtremeRevolution), but thanks for helping to keep the discussion going @Chainringtatoo.

I notice in the Amsoil specs
, which implies that the additive is not normally mixed in.

Mopar has changed the specs recently on a few items vs. the owner's manual, and I am in the process of verifying the information and incorporating it. Another thing they changed is Air Conditioning refrigerant, but I have a feeling that is an EPA thing. I feel obligated to get all the confusion cleared up, as people are depending on my Specs, since the owner's manual is so vague regarding fluid callouts.

AMSOIL also says in their spec sheet that "AMSOIL Severe Gear is compatible with most limited-slip differentials. For applications that require additional limited slip friction modifier, add AMSOIL Slip Lock." This is consistent with what I was told by their technical services staff.

The owner's manual is vague, again likely because Mazda wants you to buy their fluid instead of anything else, but this isn't some exotic differential that is so different from anything else out there. Mazda uses their cone-style LSD simply because it is more compact and lighter than a clutch-based LSD with a comparable friction surface.

I did a bit of "stress testing" in the rain in empty parking lots to see if I could get one wheel to spin freely, like a one wheel burnout, and ultimately couldn't. Any time I'd throw the back end out and hit the throttle, both wheels would spin and I'd have traction. I couldn't get one wheel to just sit there spinning while the car struggled to move forward.

If you have ramps, a quick way to tell if the LSD is working would be to drive the car up on ramps, using only one ramp on the front passenger wheel. The weight of the driver, and the forward weight bias will keep the car level, while the rear passenger wheel lifts off the ground. At that point, you should still still feel the car pulling forward, even as the rear passenger wheel spins freely.

Again, as GWR noted, this isn't a very aggressive LSD. If you're looking for maximum performance, they have a much more aggressive, and much faster reacting LSD you can order.

Here's a quick video on how limited slip differentials work.

 

[XtremeRevolution]

These video gives you a bit more information on how LSDs behave as well. In this case, his outer wheel is on a snowbank and slipping substantially, and only after there is a significant speed differential between the passenger wheel and the driver wheel does the driver wheel start to pull away.

In this case, you can see the passenger wheel start to pull, while the driver wheel is spinning in place.

 

[XtremeRevolution]

I was doing some testing with my car earlier and it exhibited similar performance to what I showed in the video above, but I didn't let it slip long enough to start catching since I didn't have the outer wheel completely off the ground (ah, that wonderful smell of tire smoke). With fluid being cold, it will take a bit longer for the LSD to engage as the clutches get closer together as the fluid thins (when it heats up). That's why you saw what you did in the video of the BMW on page 3; a ton of slip until the LSD heats up enough for the cones/clutches to start grabbing.

If you're looking for more aggressive LSD engagement, you might want to try a 75W-85 GL-5 gear oil, or just buy the upgrade available on GWR's site.

I was talking to Travis from Deyeme today about this. With both rear wheels off the ground, if you spin one, the other spins in the same direction. Great: the LSD works. If you put the transmission in drive (engine off of course), and spin one wheel, the other spins backward, but you feel quite a bit of resistance. I'd say a good 30-40 lb-ft of resistance. However, while aggressively driving the car, I never once had a "one wheel burnout." If I ever pushed the car hard enough, the entire back end would kick out.

I'll do some testing in the spring once the car is back out and the weather warms up a bit. Starting to get the impression that this LSD is not very aggressive at all.

 

[XtremeRevolution]

Getting ready to flush my diff oil tomorow. Happy to do a test if you know a good place to send it .
interesting thing is happening when the temperatures gets hot . When I am braking , heel toe shifting down into 3rd my rear end locks up and abs lamp blinks turning traction control back on. I’m thinking something is either wrong with my rear brakes , but considering it happens precisely when I downshift into 3rd could it be my diff ? Too much heat ? I am running wider stickier tires and putting down a lot more power . Any thoughts on this ? I’ll pull the oil Tomorrow and see what it looks like.

Rear end locking up? That's not mechanically possible from a simple downshift, unless said downshift broke traction. Are you rev matching pretty well when you let off the clutch into 3rd? Or does it happen precisely when you get into 3rd gear, before the clutch is let off?

What gear oil are you running now?

If you want it tested, I can help with that. Save everything you get in glass jar and I can get you an oil analysis kit.

 

[XtremeRevolution]

My brakes have been funky since I took willwoods off and introduced air into the system . Was also sent the long rear stainless steel lines and my rear pads rotors are stock with the c43 kit upfront . I’m sure all that is somehow playing a factor. Time to get normal length rear stainless lines, beef up the rear brakes and pads so it’s consistent front to rear and re bleed . Hopefully that takes it back to normal

If you have air in the system, that will cause all kinds of braking issues. Get that resolved ASAP. You are using a vacuum bleeder pump, right?

 

[XtremeRevolution]

Would you recommend adding a treatment to the manual gearbox if so witch one?

No, treatment or additive is not needed in the manual transmission.

Thanks,

Andrei