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:
SEVERE GEAR® 75W-90
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.
