I'm ready for my first oil change on my 124 spider. Both the oil cap and the owners manual states we should use synthetic 5W 40 oil. I live in Southern California a warm temperate climate year-round and 5W 40 oil is nowhere to be found except by mail order. I'm thinking 5W 40 is recommended to cover all climate zones from the south to the north. What is the opinion out there about using 10W-40?