I recommend you use the 75W90 gear oil in the axle. Amsoil has a severe duty gear oil but unless you have a heavy vehicle and tow a lot, it is not necessary.
If your engine does not use oil or leak it, use the weight recommended on the oil fill cap. Most 90's vehicles were recommended 5W-30 for normal duty and 10W30 for severe duty. I have been using 5W-30 and presently have 229K on my 92 2.8L V6. I get 6 to 8 K between quarts of oil consumption.
Get the engine flush and run it before you drain out the old dyno oil. I usually put on a brand new filter when I convert a vehicle to synthetic an drun the flush to help get the sludge and crud out. This allows the synthetic to begin working a bit faster.
Amsoil has the best filters too. They originally were rated 6 month or 12,500 miles with the 1 year or 25K miles recommended for the full synthetic. the new EaO type filters are rated for the full 1 year 25k. Both my 92 truck and 88 car will get their oil changed this week end and I have the new filters to use. The air filters are reuseable like the K&N. I have used a vacuum gauge to show as much as 1/2 inch of increased vacuum over a paper filter.
Oh yeah, I use amsoil. My 88 had 140K on it when I put the synthetic in it.