Auto-Rx the engine to start with. Then start using a good synthetic oil like Mobil 1.
With the transmission you can do a service that includes exchanging the fluid with a good synthetic. This is not a flush do not let anyone use any solvents or a machine that uses pressure to exchange the fluid. Use only a passive machine that uses the cars transmission pump to exchange the fluid.

See Brickboard Faq's
http://brickboard.com/FAQ/700-900/TransmissionAutoAW30.htm#96090FluidFlush

The timing belt is something you DO NOT mess around with.
It should be changed every 70K miles or sooner. There no excuses you can come up with not to do it. It snaps you lose the head at least. Don't get cheap replace the tensioner and pulley at the same time.

Fuel filter should be changed at 50K miles not the 100K Volvo recommends.
Put good tires on the car. Cheap tires make the car ride cheap. That one will get me flamed. The air filter is a 30K mile item same with the plugs. Both are relatively cheap to do.

These cars love synthetic fluids.


Here is the place to start the Brickboard FAQ's on 960's/S90's
http://brickboard.com/FAQ/700-900/960_90Info.htm

Auto-Rx website http://www.auto-rx.com/index.html
Several of us have used it and Auto-Rx works wonders on an engine fed a diet of mineral oil.