I use Ultra-violet leak detection in my car and the additives are always neon green. Perhaps somebody put a leak detector in with the oil to try and find a leak. I use it in my motor oil and in my coolant and there's no reason why it couldn't be used in a tranny or diff, they even make an additive for fuel systems and air conditioning. I got the kit from Eastwood. Otherwise, 20w50 is correct.