In Reply to: Re: 90 WT Mineral Oil posted by mthomas on March 28, 2018 at 09:47:26:
The only place I have ever seen the recommendation for "pure mineral oil" is in early Ferguson 9N manuals. I don't know exactly what message he was trying to convey but he was a knowledgable engineer and it surely wasn't a warning to avoid EP gear oils. More likely a caution to horse farmers not to use vegetable oils.
In any event the Ford 8N manual recommended "mild EP gear oil" SAE 80 in the winter and SAE 90 in the summer. By the late fifties/early sixties they updated that to recommend their new multi-purpose oil (M2C-134x specification) for year round use. In today's world that would be any good SAE 80W90 GL4/5 gear oil or a universal tractor transmission fluid (e.g. M2C-134D). You can use GL1 oil if you like but it falls well short of either of them in terms of wear protection and all season viscosity performance.