FSM calls for API GL-4 or GL-5 SAE 75W-90 oil for the 5 speed manual. Today I purchased Valvoline SAE 75-90 oil. Also states High Performance Limited Slip on the bottle. After getting home, noticed on the back it states
Differential: API Service GL-4, GL-5
"Non-Synchronized Manual Transmissions: API Service GL-5

The 5 speed in a 1992 truck is synchronized, correct? If this is not the correct oil, what should I buy?

I recall reading in other threads that using the wrong oil will cause excessive wear on the synchronize parts. Any truth to this?

Thanks

Thanks

 

Take it back and get GL4 only fluid. The GL5stuff will mess with the syncros and can cause notchy/crunchy shifting. Happened in mine and I switched to redline MT90 and its smooth as butter.

 

The GL-5 oils as compounded in their early days, contained ingredients that were found to corrode brass components in syncronised trannys.

Oil product suppliers soon changed their formulas so that brass corrosion was no longer an issue, but the ongoing issue with GL-4 and GL-5 spec gear oils is that the GL-5
oils have a considerably greater 'slipperiness'.

This is a benefit in differentials where there is substantial sliding friction between the gears and the best GL-5 oils should be used in them.

When GL-5 oils are substituted for GL-4 spec oils in older trannys, the increased 'slipperiness' interferes with the syncros' ability to bring their gears up to speed, and when the syncro fails to do so, gear grinding or hard shifting occurs.

Find a single spec GL-4 oil for your tranny and use the GL-5 oil in your diffs.

 

always stuck with royal purple for the differentials and GM Synchromesh for the trans. smoooooooth.

 

Redline MT90 here. Currently at 275,000 miles on the original 5 speed. She shifts like a dream.

 

mt-90 x 2. i've tried mobile 1, but still...for the price...didn't feel nearly as good.

 

Thanks for feedback. I will find some Redline MT-90. Should I just drain and fill with Redline, or is there a recommended flush process?

 

Just let it drip long as you can, overnight would be good.

No need to flush unless your old oil was very dirty, or contaminated.