80w 90 gearbox oil Synthetic or Non synthetic

[floatingjimi]

Having the gear box oil changed, whilst replacing rear oil seal Can anyone tell me if there is any benefit etc form goinf mineral or synthetic?
thanks

[jensena]

I'd just go with what the book recomends

[350matt]

The important thing is to get an oil which works with the synchromesh whilst providing decent protection for the gears and bearings.
The recommended oil by Tremec is ATF, however when I was involved with a developemnt of the T5 I spoke directly to the chap who was in charge of technical developments at Tremec. He said the only reason ATF is specified is to save cost by providing commonality of fluids on the vehicle.
ATF works well with the synchros but is awful for gear protection.
Which is why Cerbs are particularly inclined to weld their 5th gear to the shaft it runs on as there is no true bearing, only metal on metal which relies on the oil to survive.

The upshot is I'd reccommend a semi-synthetic 75W-90 like castrols Synthrax this is thin enough (once warm) to give good quality shifts and provide good protection for the gears and bearings.
Addmittedly when its stone cold the first few shifts will be a little slow and may crunch a bit but then you won't weld the rest of the box together.


Matt

Edited by 350matt on Thursday 14th December 13:41

[floatingjimi]

Thanks guys, interesting research as the top indies seem to differ, one going along the mobil shf atf the other going along the 80w-90 route, oppie oils along the atf fluid as well.

Ho hum, where is that coin