You also might try one of those engine flushes just before an oil change. If sludge is the culprit it might break up enough to let it work properly. I run a qt of kerosene (generic engine flush) before all my oil changes and I have never had a sludge problem with any of my motors since the early 70's. If it pleases you you can use that 'sea-foam' stuff, whatever breaks up sludge should be OK for the motor. Kero residuals will 'burn' off after a few miles leaving your synthetic all by itself after you change it.