Not sure - I would say that the best option is to pull it apart and re-ring it. It wouldn't hurt to try decoking it (using ed_ma61's patented vacuum hose + redex technique). It probably wouldn't be 100% effective though. You really only have to worry about this kind of thing if you never change your oil filter.
In this example, I would say strip and re-ring is the only 100% guaran-damn-teed to work option...I'm not even going to use these pistons + rings
Once I've cleaned 'em up, if anyone wants a set of low kms 5S-FE pistons in Adelaide...wait a minute what am I saying - who would want 5S-FE pistons?!!?!!?!