65 GMC half ton 4 speed questions

[Johnn R]

I have a 65 GMC half ton pickup that gets driven mabey 300 miles per year. I don't think the transmission fluid has been changed for at least 20 years (shame on me)! So anyway I want to change it, but I don't have an owners manual advising what type of fluid goes in it, or what the capacity is. This is a 4 speed tranny. Can someone advise what weight and spec of oil I should use?

[blackdiesel]

90w. gear lube. dont know how much it holds, but fill it till it runs out the fill hole. We have trucks on the farm that we use all the time, and the 90w has never been change in. If it was me I would check it and the rearend, and if it looks good (no grit or water) I would fill it and let it be. BTW welcome to the site...

[dwcsr]

In your neck of the woods I would get an 80W-90 not just a staight 90. if you drive it in the winter with straight 90 it will shift very hard because the oil gets very thick at 32 and below. the 80w-90 acts like 90 in the warm weather and 80 in winter and does not get thick like the straight weight oils. gear oil has sulfur and lithium in it and over time can etch breaings and gears when it collects moisture. I would drop the oils out and refill both trans and rear.

[Wrenchbender Ret]

Those don't have to be changed very often but after 20 years I would. Also the rear end grease should be changed too. The trans. will hold about 6 pints & the rear end 4 pts.

[amccomb]

I have a 66 half ton 4spd, I am going to change the fluid and I am wondering were the drain plug and fill plug are?

[dwcsr]

Originally Posted by amccomb

I have a 66 half ton 4spd, I am going to change the fluid and I am wondering were the drain plug and fill plug are?

The fill should be on the side half way up. The drain may be just the lowest bolt in the rear cover or a plug in the bottom.