@My78truck,
If your truck is earlier than late'80's and back do not use synthetic's due to the type of lubrication is not compatible with the kind of cam and lifters those engines use. They cannot get the proper friction reduction at the point of metal to metal contact...period. Unless your engine has roller lifters or you built your engine with rollers in it that previously had hydraulic lifters, do not use synthetic. It's not better for your engine just because it's newer.. Synthetic oils will wear you hydraulic cam and lifters right out in half the time if you had used regular SAE oil. It's the lubrication properties of the oil it is, SL, SJ, SM are all fine, and if I can dig up that paper there are other oils than just these. I contacted a oil "engineering" company because I wanted to find out what I should use and they sent me back a detailed report on current engine oils and trans fluids to. The paper explained what certain oils do and why. It was really interesting, if you are into engineering reports about oil. It gave you an idea what it looks like right where the lifter metal comes in contact with the metal surface of the cam goes through with conventional oil and synthetic lubricant.
Do not use any trans fluids other than Dextron III or Mercon or less, in any TH350, TH 400 or 700R4. If you use Dextron/Mercon V and up it will destroy the clutches in your transmission due to the fact that the abrasive material on the clutch disc is made up of certain compounds for the even wear and heat resistance, wear properties and compatibility with certain oils. Newer transmissions are not in any way like a TH350 and take a completely different fluid than older transmissions. It has nothing to do with the metal, it's the clutch disc's. Clutch disc's that can withstand newer transmission fluid don't exist so you have to stay with fluids for older transmissions. If you have a 4L60-80 new fluids are fine.