1950 D6 8U transmission oil

I am new to the forum but was servicing my D6 and found the transmission oil to be milky, where would water get into the transmission oil? What oil is recommended for the transmission I have read anything from 30wt. engine oil to 90wt. gear lube any recommendations for operating in central Illinois. Does anyone know what size the diesel fuel cap is on this dozer I thought it was 4" pipe but is slightly larger and I cannot find one.
Thanks,
Brian

Brian,

One place water gets into the tranny is through the boot on the shifter. Even with a canopy it can get in. I put an empty bucket upside down on the shifter. .even after I've put a new boot on.

As far as oil for the tranny, I'd go with 80W-90 oil there and in the finals.

For the fuel cap, you'll probably wind up going to Cat unless you can find one used. Just grab your ankles and back in the door 😆

All the oils you mention have been used in CAT equipment. At one time (1970's??) 30 wt was acceptable in everything. As far as I know that has not been the recommendation for a long time now. The savings of not having to have more than one oil on a service truck etc are likely to be far outweighed by the maintenance savings from running a lubricant in each component that is ideal for that use.

While there is some overlap in the viscosity of gear lubes and motor oils and the actual difference may not be as great as the numbers would indicate, the additive package of a gear lube, a hydraulic/clutch/brake fluid and a motor oil is likely to be very different.