Can I run a 20 offset on my R50? I know the average is around 40. What difference will it make? Or can I get wheel spacer to make up the difference?

 

It depends on how wide the wheel is as to whether it will fit, and you'll be changing the scrub radius. Adding spacers will make fitment worse since you'll already be moving the wheel farther outboard by lowering the offset.

For reference, the stock 15x5.5" wheels are ET45, and the 16x6.5" and 17x7" wheels are ET48.

 

They are a 16x8.

 

That's going to be close. I suspect you'll end up hitting the arches in the back if you go with a stock height tire, and they will definitely stick out past the fenders.

Actually, here are some 16x8 ET20 on an R53. It seems to confirm what I suspected, though that car is lowered.