I have a question about what differential oil and additives I should put in them. I have the G80 limited slip diff and don't know what additive to put In the front and back. I have 75w90 synthetic I will be putting in the back and 80w90 synthetic in the front. i also notice that in my repair manual is telling me to fill them only to about 5/8 to 1 5/8 inches. is this true or can i just fill it until the oil is even with the plug?