Owners manual says either GL3 or GL4 oil can be used and it should be 75w90 and 2.1-2.25L is required. Question is what is the best gearbox oil to use for the 659cc Copen? And Can GL4+ be used? (reason for asking is Castrol Syntrans Transaxle 75W-90 was recommended but it is GL4+ rated and the description says it is compatible with GL4 but I want to know for sure)
You have already answered your own question. Use Castrol Syntrans Transaxle 75W-90. It is specified as GL4, the + is a marketing term used to indicate a higher amout of extreme pressure and anti wear additives.
I changed the fluid and it went well. Gearbox feels a bit smoother then it did before (might just be in my head) I didn't have a long extension bar to be able to reach the fill plug through the wheel well, but I did have a pretty long spanner and I could just reach the fill plug from the engine bay and I could remove the drain plug if I crawled under the car with it raised up. Here is the part number of the gasket for the drain and fill plug incase anyone needs it: 90044-30178-
