T138 transfer case recommended oil ?

[zxmedic]

Hi guys I've tried the search and no luck tried oil change , oil etc etc no find ..

What are you guys using for oil I have the Rockwell service manual here and it clearly states to only use GL 1 90 weight oil which is a mineral base and to not use higher gl rated oils as they are not mineral based . Has anyone found a proper substitute

 

[Scar59]

Just filled the tranfer of a M939, the LO called out 80/90.