Multi purpose light weight oil with advanced additives has been replacing the old heavy oil for 40 years at least. Only drawback is if you have leaks it will leak more and if you have gear wear your gears may be noisier. Good part is a common oil to stock and in the winter hyd and shifting and steering and all that work better. On one of my Fords my hyd line goes through the bottom of the tranny and has pin holes. As a result I have to periodically drain back down to the fill hole. So, this tractor has been running Ford 134a (multipurpose hyd. tran. fluid) rather than 90 w in the tranny for about 15 years that I know of.