What is the thinnest oil that would work at least somewhat adequate in a 8.25 open diff in a light weight very low power kind of car? Just thinking about doing some more MPG experiments and was wondering if I could use something lighter like slick-50 or maybe ATF+4 since they use it in FWD auto tranns with differential gears in them. Not nessacarily looking for 200,000 miles out of it but if I could get 50,000 - 100-000 miles out of one.