I have found there are 2 other places that might need attention, but the best bet is the STC is leaking - kind of internal bleeding, the oil leaks into the crankcase. This is probably the highest potential for slow loss of performance, and the other is the o-ring under the STC fitting, where it bolts to the branch tube "block". I had this problem and I tore the top of the motor apart 3 times before I found it. Be sure to replace this when you do the STC, I don't think it comes with the STC "kit", but almost everything else does, including a new screen for the IPR. Don't use the plastic bolts or the plastic oil sling sheild that comes with the kit - you can't tighten the STC bolts enough to get it to seal, the plastic shield breaks. Also be sure to replace the o-ring on the test port plug on top of the HPOP