I just bought some Valvoline 80W90 for my rear end. I read the label and it said it is for rear ends and non-synchronized manual transmissions. I had an older bottle of the same stuff and the label was virtually identical but didn't mention non-synchronized manual transmissions. What gives? I can't put this in my Muncie to top it off?

It has something to do with GL-5 grade. It eats at the brass synchronized gears. You need GL -4 for brass.

Thanks. I looked at the older bottle and the newer bottle and they both say they meet GL-4 and GL-5 though only the newer bottle has the "non-synchronized" wording. Something they recently figured out?

I don't know when it was found but here is some technical info.

Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

EP additives which contain phosphorous/sulfurous compounds are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizers; the GL1 class of gear oils does not contain any EP additives and thus finds use in applications which contain parts made of yellow metals.

GL-5 is not necessarily backward-compatible in synchro-mesh transmissions which are designed for a GL-4 oil: GL-5 has a lower coefficient of friction due to the higher concentration of EP additives over GL-4, and thus synchros can not engage as effectively. Also, transmissions which explicitly call for GL-4 oil may have been designed around this lower concentration of EP additives and thus may contain yellow metal parts which GL-5 will corrode


That being said, I choose to use
BRAD PENN®
Multi-Purpose 'Classic'
GL-4 SAE 80W-90 Gear Oil

The SAE 80W-90 is specially blended using high quality PENN-GRADE® mineral base stocks and select additive technologies. It is designed to be used with “yellow” metallurgy (brass, bronze, copper) and other ‘soft’ metals used in synchronizers, bushings, thrust washers and other components typically found in classic manual transmissions and transaxles. In applications specifying API GL-4 quality lubricants, use of API GL-5 lubricants with higher extreme pressure additive activity/concentration can damage these ‘soft’ metal components.

Thanks for the info.

Good info indeed.
I just checked to see what I had in mine. Same bottle most likely that Bob has, Valvoline GL-5 with no mention of synchro trannys.
So, I reckon it is time to drain and change out the oil. I forget, is there a drain plug on the bottom of the case to allow for draining it?
Cal

I run Mobil 1 synthetic in my Muncie.

No drain plug. You have to suck it out somehow

Most new gear lubes contain a friction modifier for use with limited slip differentials and it's not compatible with the synchronizers in a manual transmission. It's too slick and prevents the synchro from grabbing the cone and and slowing it down enough that the synchro teeth mesh allowing a smooth gear change.

If you have a manual trans, find a GL-4 lube that's free from friction modifier and you'll be just fine.

Thanks Rocky,
Cal