I was planning on changing out the gear oil on my rear differential. Nothing wrong, just preventative maintenance since I have no idea when it was done last. The car is a totally stock 1963 Galaxie 500 Sedan with 352, Cruise-O-Matic, and 3.00 rear end. What gear oil weight should I use? The manual recommends straight 90 but that was obviously before synthetics existed. Car is a semi-daily driver. Thanks in advance.