I would appreciate some thoughts on what type of gear oil I should use in a 1989 F-150 4x4. It was my Dad's old truck and I have started driving it. It has 122K miles, but looks great and runs great. I am using it here on my farm .... just like my Dad used it. The manual suggests a Hypoid gear oil for the front and rear differentials. It lists a Ford Part #, but doesn't give a weight or even a range of weights. I have only used Amsoil products 1x in my life, but I purchased some Amsoil SVG (Severe Gear Oil) for my 2004 Tahoe. I thought it was a great product and might use it again. I will need about 3 quarts. Question: What product would you guys suggest for this old Ford? I would like to keep it running as long as possible. My young boys love riding in it. I will use it to pull a trailer and haul hay, so it will be a work truck. Thnks.