An ice age and three-quarters ago, I used to rebuild Fuller (Road Ranger) transmissions and auxiliaries. The factory insisted on using 50 weight mineral oil and nothing else. Why? The tight tolerance bearings. The counter-shaft ball bearings would gall and seize if 90-weight was substituted.
This left me very leery of migrating away from OEM recommended lubricants in the drive train, unless without question 90 weight GL5 gear oil was factory specified.
The dealer parts department or service manager would be the most reliable source of information for you.
As an aside, I got totally different answers as to the correct fluid for my MoPar automatic transmission from four aftermarket sources. None of them happened to agree with the original factory agency service department.
A different colored and consistency lubricant would definitely raise my eyebrows. With a component as expensive and as hard to fix as what you are dealing with I would put forth effort to ensure whatever I did would not cause failure.
PS: Mopar says the ATF4 fluid in my transmission is not "vital" but firmly suggests transmission life can be markedly prolonged by using it and nothing else.
