I'm going to change the gear oil in my front and rear differentials in my 85 trooper soon but I don't really know that much about gear oil. Which brands do yall think are the best? More than likely I'll go synthetic unless its not worth the extra money. I need 75w 90 for the front and 80 w 90 for the rear and I was wondering if I could just use the 80w for both. Would it cause any problems?Also, I don't know if I need to add the limited slip stuff or not, do I?