Hi,
I have a 1989 MX-6 GT (165K miles) with a manual transmission. I'm not sure that the oil has ever been changed. There is no problem other than shifting has become more difficult over time. Question: would it be a good idea to change the transmission oil? If so, what should I use (RL MT-90, standard 80W-90)? Someone in the Mazda parts department suggested that changing the oil that has been in use for so long could be a bad idea. The gears would be use to the thick old oil and changing it could created problems. He suggested just keeping the oil at the proper level and do this by using 80W-90.

Thanks in advance for all advice...Bill

 

I don't see where changing the gear oil would be a bad idea... especially w/all those miles on the car.
I have had good luck using synthetic oil and gear oil over the last... 25 yrs?
My recommendation would be to switch over to a light weight synthetic, a 75w-90 if you can find it, but nothing heavier than recommended in the owners manual.
Remember that the second number is actually the weighted viscosity of the fluid in question. The first number is only how it will "act" in winter conditions. ("w" stands for winter.)
Keep your eye on that second numbermore thanthe first.

 

I change trans fluid at 50,000 miles, regardless of the type of vehicle or type of fluid being used.