The Stovebolt Site Forums The Parking Lot '60-'66 Trucks 90W Gear Oil Alternative

I need to replace the oil in my front differential and in the transfer case of my truck (1961 K10 Pickup). The manual calls for straight 90W gear oil, but it's proving to be hard to find. Are there suitable / recommended alternatives?

Lots of hits saying that 80-W90 is the same, but others saying to stay away.

Thanks!

- troy

Tractor Supply Co. 90 weight Ford tractor transmission oil is a direct replacement for the straight mineral oil specified for the transmission and transfer case. It does not have the hypoid additives that supposedly attack brass synchronizer rings.
Jerry

My NAPA also gets it for me, but it is not in the store.

Thanks - I have a Tractor Supply close by and I'll go check there. Much appreciated!

Do not use the straight mineral oil in the rear ends. They must have GL-5 gear lube. 80-90 will work fine.
George

Ditto- - - -the straight 90 mineral gear oil is for transmissions and transfer cases only.
Jerry

I’m glad I read allot of these post, otherwise I wouldn’t know I just screwed up and just installed straight 90 weight mineral oil in the rear end. Fortunately I have not driven it yet as I still need to install my engine.

Do I need to flush it out or is draining sufficient?

Phil, just a drain and change is all you need.

Doug

The difference is in the design of the gears. They're called "hypoid gears". The pinion gear in a drive axle is below the centerline of the ring gear, and the teeth mesh with a shearing action that requires a high pressure lubricant. The hypoid additives in some gear lubes supposedly can damage brass synchronizer rings. A rebuilding depot for Fuller Road Ranger transmissions that I used to do business with in San Francisco had 55-gallon barrels full of scrap synchronizer rings with heavy black deposits on them, and the manager of the facility told me they came out of transmissions that had been running "multi-purpose" gear lube with hypoid additives in them.

https://www.motioncontroltips.com/hypoid-gearboxes-what-are-they-and-where-are-they-used/

Jerry

Since we are talking about rear axle lubricant...
If the rear end has a "positraction differential lubricant only" tag, you are supposed to put in an additive. Is the additive strictly necessary? What does the additive do?

The additive prevents, or at least reduces the tendency for the limited slip clutches to grab and chatter in low speed turns. Without it, some types of locking rear ends have a noticeable shudder in turns unless enough throttle is applied to make the clutches engage fully. On dry pavement with good tire traction, limited slip axles can get pretty noisy in tight, low speed turns without the additive.
Jerry

Thanks!