The Stovebolt Site Forums The Shop Area Driveline Transmission/rear gear oil

I know this has been discussed in multiple threads. I did a search and found a 100 different suggestions/answers. Last time I researched this there was a Tractor Supply mineral oil recommendation that appears to no longer be available. All of the old threads I looked at with recommendations had broken links that no longer go to anything. Bottom line, my 54 3100 has the original driveline. Is there a product safe to use in both the 3 speed transmission and rear end? Most of what I am seeing is GL-5 type 80W-90W

I use 75-90 weight gear oil in both.
The royal purple stuff if you can.

I'm pretty sure NAPA has 90 weight straight mineral oil without the hypoid additives which is the preferred lube for the transmission. That should not be used in a rear end, as it does not have the film strength to be used where the spiral loading of the ring and pinion gears are involved. Hypoid lubricant in a transmission has a tendency to have an unfriendly reaction with the brass synchronizer rings and thrust washers. The ultra-slippery synthetic lube does not play well with the heavy gears and high inertia of the SM-420 transmission, resulting in poor synchronizer performance and gear clashing during shifting.
Jerry

Do you use the Royal Purple in the stock 3 speed tranny? Royal Purple is the only one I've seen that specifically says on the label that it won't harm yellow metal.

Last I knew, Tractor Supply still had it.

Edit: I think I linked the wrong product. Use Jerry’s.

Same link I just found, I believe:

https://www.tractorsupply.com/tsc/search/ford%20tractor%20transmissionfluid?

For the rear axle, use any brand of multi-purpose 85-90 weight hypoid lube.
Jerry

Do you use the Royal Purple in the stock 3 speed tranny? Royal Purple is the only one I've seen that specifically says on the label that it won't harm yellow metal.[/quote]

No I use it in M-20+22

Thanks for the follow up. I will be getting mineral oil for the transmission and use the 80W-90W for the rear end.

Very minor point. You will get hits from Googling "80W-90W" but they take you to "80W-90" items w/o the 2nd W. I think the 2-W version would be impossible.

The "W" in an oil specification stands for "winter"- - - -its flow viscosity is measured at a low temperature, possibly zero degrees F., IIRC. For instance, an 85W-140 synthetic gear lube flows like 85 weight at low temp, and lubricates like 140 weight when hot. Engine oil gets the same type of rating- - - -flows well when cold- - - -lubes like a thicker oil when hot. NO- - - - - -the physical thickness doesn't change- - - -5W-30 engine oil stays watery thin regardless of temperature.
Jerry

My mistake for adding the W but that is interesting information. Something I did not know. It never crossed my mind that W stood for winter. I am always happy to receive this kind of feedback.

For the longest time, I thought the W was for "weight."

Not quite right. 5W-30 is the SAME viscosity as a straight SAE 30 oil at engine running temperature. It's also the same viscosity as an SAE5 oil at the lower temperature. If you want to call that watery thin, then OK. The rating temperatures for oil viscosity is 0C (32F) for the W viscosity and 100C (212F) for the higher viscosity. Viscosity index (VI) is a measure of how close the high and low viscosity are at the test temperatures. An oil that's the same viscosity at both temperatures would have an infinite VI, and doesn't exist as far as I know.

You can define "watery thin" however you want, but to be clear multi-weight oil gets thinner as it heats up.

Take a bottle of 10W-40 and put it in the fridge, then put another one next to the stove while you make dinner. Pour them side-by-side and the one at 100°F will be thinner than the one at 34°F.

I think that is what you meant, but your phrasing was somewhat confusing.

Kevin, that doesn't agree with the timed tests I've run on multi weight oils, using a container with a precision drilled hole, a stopwatch, and an infra-red thermometer. What have your test results been like?
Jerry

Jerry, the API tests and the definition of the viscosity ratings are all that's needed to defend my statement that 5W-30 has the same viscosity as straight 30 weight oil at the higher temp test point.
Yes, the multi-weight oils have close to the same viscosity at different temperature ranges. Are you saying that straight 30 weight oil is too thin (watery thin) to use at normal engine temperatures? Single viscosity oils were all that was available way back when the AD series trucks were new.

I thought so.
Jerry