Differential Grease

I have been told that I can use STP in the differential instead of 600w. I dont want to ruin the differential

Larry

I've heard that rumor myself, but I can't see where that would have better results than what is recommended.



I *have* heard of people running 90-140 in their differentials, but they had to do some kind of seal work, as it (supposedly) tends to ,want to migrate out of the axle shafts and up the driveshaft housing, putting it where you don't want it.



On the other hand, Ruckstells use a different grease (I saw it on someone's websie, I think it's 90w or 90-140, but the gearbox apparently needs the extra fluidity).



Me, I use 600W... I used to get it from a local old car parts place (Mal's A sales in Martinez, CA) but last time I checked I couldn't find them... maybe out of business??) I would assume any of the big vendors would have it (I hope I can find some anyway!!!). I've heard you can get it through industrial supply places and lube jobbers (esp. if you're in "farm country&quot but I don't know for sure.

Check the telephone Yellow Pages under "Oils - Lubricating". You should find distributors for most major oil companys.

All should have 600W available. If they only want to sell to you in bulk, request names of their customers who will help you with small quantities.

STP is a viscosity increaser - it makes lubricants thicker. Why not use a more viscous lubricant to start with?

There are widely different opinions about which lubricants are best in vintage cars. The 600W lubricant is a steam oil cylinder oil (yes, for steam engines). The 600 value is not the viscosity, but the flash point of the oil. While it is very thick at room temperature, it thins out greatly in use as it heats. Many knowlegable Model T hobbiests use modern gear lube- 140 wt in regular Ford differentials and 90 wt in Ruckstell 2-speed axles. Gear lubricants are usually found in multiweight formulations (90-140) which would serve both uses. I understand the gear lubes can be found in single weight formulation, but not at your local auto supply store. One more item of disagreemenet is the use rating shown on the container. GL5 rated lubricant is for modern cars and contains additives that can be harmful to brass or bronze. GL4 rated lubricant does not contain these additives. How harmful is the GL5? One report I saw, by a person in the petroleum industry, thought it would be decades to cause appreciable damage. As I said, opinions vary. The final choice is yours.

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I had a problem with differential oil leakage from the rear wheel bearings, and after putting in modern seals, etc, etc, learned from our local Guru, Fred Houston, that the proper diffie oil level is well below the filler plug: "just enough to barely be able to reach it with your pinkie finger".



As as the diffie oil lubes only the diffie and the inner axle bearings, only enough oil to reach the ring gear is needed; the ring gear will throw it around enough to lube OK, similar to the flywheel lubing the tranny.



It's easy to forget that the outer bearings on T's are lubed by the two grease cups at the ends of the axle housing, and if enough oil is in the axle housing to reach them, they'll leak.



Back in the T era, there were many fixes sold for rear axle leaks; imagine this was because folks were overfilling then, too.