Is 75w-90 recommended to be used in our older 1987 Toyota Vans with synchro manual transmissions? The manufacturer's repair manual for these vehicles indicates that the manual transmission and transfer case should use GL-4 OR GL-5 75w-90. Your website instructs that it is safe to use in GL-5 applications but I've been told that it may not be recommended for manual transmissions designed for GL-4. Since our repair manual shows both GL-4 and GL-5, and we know that these old syncrhos have some plastic or soft metal parts that can be broken down by some synthetics, is it considered safe to use Severe Gear 75w-90 in the Manual Transmission and Transfer Case in 4wd 1987 Toyota Vans? Also, our specs for the differential call for SAE 90. Do you recommend the Severe Gear 75w-90 or 80w-90 gear oils be used in the differentials for this same vehicle?
And for complete clarity, do I understand that you are saying that the SevereGear 75w-90 gear oil is not harmful to the manual synchro transmissions or should I use something like the synchromesh transmission fluid 5w-30 in the 1987 Toyota Van 4wd?