Any other gear oil other than straight 90w mineral based oil will cause the synchronizers to clash. The ash content in the modern multi- grade gear oil is too low, which causes the synchronizers to slip and not engage the gears correctly. Ford tractor straight 90w is GREAT. Napa also carries a 90w straight mineral based gear oil, but you will need to ask one of the people behind the counter for it. The bottles will probably have a heavy dust layer on them. Don't think this stuff is a hot mover.
