Slick 60's

I have a 65 f-250 with a T-18 tranny. The shop manual says to use 50w oil.

Can i use 80w-90 with out any problems?

I just went through this dilemma myself within the last few weeks. I decided to try Castrol Syntec 75W90 when I reinstalled my NP435... we'll see how it works.

Cheers,
Dion

is that a synthetic? i worry about using a synthetic on my old seals.

I run 50wt motor oil in my T-18 and it works well.

mr. c - yeah, the 75W90 is a synthetic. I thought I'd give it a try based on a bunch of reading I did.

As for the 50wt motor oil option, I know it's what the manual called for way back when... but after having a conversation with a few guys, I can't see how engine oil - which is formulated with the intent to protect but drain off components very fast, is better than any sort of gear oil that is formulated with certain "climbing" properties. Having a film of the gear oil remain on the gears, etc. makes a lot of sense to me. That plus later on the NP435s called for gear oil if I remember what I read.

Cheers,
Dion

so where can you find non-detergent 50w?

Years ago, I faced the same dilemma. There is a wonderful write up over at www.bobistheoilguy.com

Turns out engine oil and gear oil viscosity are rated DIFFERENTLY.

50w engine oil is about the same viscosity as 90w gear oil.

Makes no sense, I know. But that's the facts...

Put the 75w90 in there....

I confess -- I learned this after I located some 50w racing oil and put in in my tranny. Didn't hurt the tranny...

Finally, I agree that I would be carefull with synthetic oil in a t-18 that has old seals. Syn oil does NOT cause leaks. But if you have one, it will leak more. At age 50, I am sure that many t-18 trannys have some slow oozes. Best leave it to dino. That tranny will not fail anyhow, unless it has NO oil in it...