1967 GMC 930 Manual Trans Fluid?

[supertrooper]

The manual calls for “straight mineral oil gear lubricant”. I am not finding any locally. Does anyone know if there is a modern replacement for this?

[RustyPile]

My Chevrolet Manual says SAE 80 or SAE 90 GR5.... Same for all manual transmissions.

['68OrangeSunshine]

Originally Posted by supertrooper

The manual calls for “straight mineral oil gear lubricant”. I am not finding any locally. Does anyone know if there is a modern replacement for this?

Any non-synthetic gear oil will work. For your latitude, a viscosity of SAE 75 to 95 will work. By ''Straight'' they seem to want a single viscosity, not a multi grade. SAE 90 might work. 80W - 140 would be good for the Desert Southwest.

Also if you have an SM 420 transmission, with brass synchros, you want service GL-4. GL-5 has additives for limited slip [Positraction] but these additives attack brass synchros.
''Mineral Oil'' refers to natural petroleum oil, not Synthetic. In 1967 synthetic lubricants were rare. Much has progressed in this field since then, but if you want to go by the book, avoid them.

[supertrooper]

My dad was doing some reading and came across something that said to make sure there is no sulfer in it…?

[1970cstblazer]

GL-4 80w90

[supertrooper]

Looks like this is what I am looking for. My dad did some more checking found someone that also recommends the GL-4. Most is GL-5 and not what the transmission wants.

Here is a quote from Penngrade oil.."PENNGRADE 1® CLASSIC GL-4 GEAR OIL SAE 80W-90
PennGrade 1® Multi-Purpose ‘Classic’ Gear Oil GL-4 SAE 80W-90 is a specially blended using high quality premium 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."

This makes perfect sense and I am confident now to replace the fluid with a GL-4.

Thanks!

['68OrangeSunshine]

Originally Posted by 1970cstblazer

GL-4 80w90

Sounds good. Why was GM leery of multi viscosity 57 years ago?
They call for ''Straight'' oil.

[kwmech]

Originally Posted by 1970cstblazer

GL-4 80w90

Right there. Have found out a couple of times when a customer has had their vehicle serviced elsewhere and then starts experiencing shifting issues. Drain and fill with GL-4. The GL-5 is too slippery for the brass rings to grab the gear and slow it down

[jumpsoffrock]

Originally Posted by kwmech

Right there. Have found out a couple of times when a customer has had their vehicle serviced elsewhere and then starts experiencing shifting issues. Drain and fill with GL-4. The GL-5 is too slippery for the brass rings to grab the gear and slow it down

Where do you pick up your GL-4, mech?

['68OrangeSunshine]

Supertrooper: What transmision do you have?

[1970cstblazer]

I just picked up a bottle of Valvoline GL-4 at my local Carquest. It was $13.