T-18 trans What oil to use?

[OLYELLER]

I can't find two people that say to use the same thing. One says 50w, one says 75w 90 gear oil. and another say 80w 90 gear oil. I'm in Chicago but never drive in the cold. Just trying to find out what to use. thx

[36truck]

75/90 or 80/90 works.

[Thunderboy]

I don't have access to what that trans originally called for, but almost all manual transmissions in those years came with a single-90 weight gear oil. Not all would agree, but I would use a multi-weight oil because it has a broader protection range. For example, a 75W means it flows at the same rate at winter temp (32 deg, I think) as a straight 75 weight, but flows as a 90 weight at summer temps. So a multi-weight (viscosity) oil should give even better protection than the oil it originally came with.

Short answer - either 80W-90 or 75W-90 would be fine.

Since one doesn't change gear oil often, I would recommend synthetic gear oil, such as Valvoline or Castrol, for even better protection. Yes it is more expensive, but studies show synthetic to work better at extreme temps and it lowers friction. Personally, except of my racecar, I don't use synthetic oil in my engines (too expensive for how often it gets changed), but I do run synthetic in my transmissions and differentials because they don't get changed but every couple of years, at most.

My $.02.

Drew

[cdherman]

Whats also not commonly known is that engine oil and gear oil viscosities are NOT the same.

50w engine oil is about the same as 90w gear oil, in terms of viscosity.

Makes no sense, but I am 100% certain on this one......