Thanks for your guys replies. I did an oil change a couple days ago... And I was amazed at the difference! I knew that it needed one (I last did it in the fall I think???), but it apparently needed it a lot more than I thought... The car is smoother, quieter, and most surprisingly, faster!
Seeing how much of a difference a simple oil change made, I'm wondering what kind of difference just doing "advanced maintenance" would do.

OM616: Thanks for your reply. I've been reading on the forum, and it seems that unless you're ready to re-engineer the whole engine, you have to be careful with what modifications you do...

gear-head: My Mercedes-savy friend has been saying that I should check the timing chain. This is the kind of car that if you haven't done it, then it probably hasn't been done...

So I'm thinking that I'll probably replace the injectors with OEM or something similar.