1991 Toyota MR2 Base 2D Coupe
The tires and alignment can effect your millage. Did the dealer check long term fuel trims? This is usually the first step when diagnosing a problem like this
A: Oil change w/different weight oil?
No but you might loose a little fuel efficiency or something like that. It's far more important to just not run low.
Shouldn't effect the steering at all. Check the pump.
A: How do I get better gas milage??
Keep the fluids clean and proper, tires inflated and keep your foot out of it. That's about it. You can have the engine checked for fault codes and check the fuel correction trim if you really want to make sure. The maximum millage you should get is 25-27, and that in an optimal situation. 22-23 is about right for reality unfortunately. Remember you are driving a heavy truck now, not an economy box.