T10 Fluid

[KR500]

What transmission fluid are you running in a street driven cast iron T10?

[Royce Peterson]

[s2ms]

Redline 75W90NS: https://www.redlineoil.com/75w90-ns-gl-5-gear-oil

[Royce Peterson]

I would not use synthetic grease like the redline product. Reason is the synchros will not be up to speed - the grease is too slippery and thus not competent for use. Really the OEM lubricant is best for a variety of reasons.

[98SVT - was 06GT]

Use something with a GL4 rating. GL5 could eat the syncros depending on what yours are made of. I'm pretty sure 90W was the original rating. I agree no synthetic. I did some homework on this when I got my Model A. The PO had used synthetic and the syncros in my 39 trans and overdrive were too slippery to engage without some grinding. Changed to orig recommended fluids and all is good.

https://penriteoil.com.au/knowledge-centre/Gear%20Oil%20Industry%20Specifications/83/what-is-the-difference-between-api-gl-4-and-api-gl-5-gear-oils/1036#:~:text=It%20is%20a%20common%20belief,especially%20those%20that%20use%20synchromesh.

[s2ms]

I was leery of the GL-5 as well so did a bunch of reading on the subject. The takeaway IMO is the argument to use GL-4 vs GL-5 in trannys is not black and white, and the argument that GL-5 might eat synchros is overblown. I was using a GL-4 but decided to try the Redline GL-5 which is a different formulation than a GL-5 used in diffs. All I can say at this point is gear engagement is definitely firmer than with the GL-4, and the tranny seems more quiet.

[trotrof1]

Penngrade 80w-90 GL4

[FL SAAC]

Several trains of thought,  you have the purist, the economy minded and the one that wants the very best.

Back in the day they packed them with regular gear oil and they lasted or last forever.

Put what ever you can afford to put in, the one that is on sale or the premium product. All of them are good. See you in 60 years

[KR500]

Thanks to everyone for the info