A couple of good strong pearls for medium oil are
MoRich's NSane LevRG and the Black Widow Pearl (I think, I'm still catching up on the new stuff so I may be wrong). I have the NSane and for medium to medium-heavy oil its great, but once I get into heavier stuff it just skids way too much.
I'm also looking into upgrading my equipment and am currently looking at heavy oil and light oil balls. I have my heavy oil choices narrowed down to the Black Widow Bite and Lane #1 Buzzbomb, and my light oil choice down to the
Roto-Grip Neptune. I got a free White Dot in a league I was bowling in on Tuesdays so that's becoming my dry/spare ball. I'm going over to the
Pro Shop tonight after work to talk to the guy about it more and see what he recommends.
There's a lot of other stuff out there, and a guy who's quickly becoming a pretty good friend in my Wednesday league is looking at the Secret Agent. Really nowadays its almost like you can't go wrong with any ball you choose for a certain condition as long as you can figure the ball out, and it seems more like its just about the ball you want or brand you prefer.