Just got an NB with 140k miles. I have reason to believe that the previous owner put Mobil 1 75w-90 GL-5 hypoid gear oil into the transmission. The trans does not grind, but shifts notchy when cold and I can feel the dogs on the gears engage as if the synchros are not gripping properly. Shifts "okay" once the trans is warmed up. Is there any chance of permanent damage to the synchros? The car is parked for the winter for now and I plan to drain and re-fill with Redline mt-90 and probably drive/flush then re-fill again to get rid of that GL-5 gear oil. Is this thing screwed?? Would it be possible to do just a synchro replacement?

 

Yes, GL-5 is not good for synchros. Dump the fluid and hope for the best.

 

Thanks. I only caught it by chance when I went to his house to pick up some parts that I did not get with the car initially. We were talking about the rear end in the vehicle I traded him, told him I used mobil 1 75-90 and he pointed to his bench and said he used that in the trans/rear end of the miata. Then my eyebrows went together