Well, there are Dingwall SJ/SP strings. You can contact Sheldon or Dingwall Sales for more information. DR strings also fit, I believe

My Super P/J came with Thomastic flat wounds and I love 'em.

My SJ5 (one of the early batches) came with DR Lowriders, which are pretty good. They come with specifically made Dingwall strings now. I think that most strings will fit 35" scale basses these days, so there are a lot of choices and it should be easy to experiment.

I was running DR Hellborgs plus a Dingwall B string on the SJ's. Hellborgs are still my favorite four string set for nickel round wounds and a very affordable $25 or so. Keeping the Hellborgs balanced with the Dingwall B was challenge, so I settled on the DR nickel Low Riders for the five string set. I bought some DR nickel sunbeams at the same time, but I have only checked them for fit, (they will) I have yet to put them on. Just this morning I put a used set of TI Jazz flats JF 344 from my last dearly departed Fender J bass on the SJ-4. They fit fine, if a little long on the E. If I like it I may go with the five string set for the Super P/J 5. I always liked the slinky feel of the TI's, tuned down a half step they are nice and loose, very soft feeling. I don't do stainless steel so I can't help with those choices. Things keep on like they are going and I won't have rounds on anything except the six string. Might have something to do with lucking into a upright bass to go with a jug band gig.

i went with the dingwall steels they sound really good on this bass. i like them