I have a few questions on rear and front differential oils for a 1991 Toyota Corolla Alltrac AWD DXL, 5sp transmission with the 4A-FE engine. My owners manual doesn't specify if I use GL-4 or GL-5 oil. It only mentions oil weights that can be used, which are 75w-90, 80w-90 or 90? I have an open rear differential, so with that is it ok to use oils with a limited slip modifier already in the oil? Because that's all I can find in any of the stores in my area. Do the transmission and rear diff oil weights need to match? Lastly. my rear diff has chalk mark on it with a 75 right above the fill plug. I assume that was the oil used. Any feedback would be great. Thank you.