The Prius was/is driven by a kid and he hasn't taken care of it. Around 150K it started losing some oil. Kid let the oil ran low a couple of times (Twice the red triangle lit up briefly when at the bottom of a steep hill briefly, so the oil was sloshing around just enough to not reach the pickup, presumably, for a few seconds). Told him to top it back up after it happened, both times. The bottom of dip stick has varnish marks on it. It's now 180K. I just did an oil change and the oil came out very dark; it's been baking in there for too long. I did an oil change and put in Penzoil Platinum High Mileage Synthetic 5w-30 & an OEM Toyota filter. As this was before I came to this forum, I used Seafoam in the crankcase at slightly higher than the recommended rate. With 4.2 qt volume there are about ~6-7 oz in there. It's been running for about 338 miles. I was hoping it would clean up whatever sludge/vanish has developed. But now I know that Kreen would have been a better choice for what I was intending. Two questions: Do you recommend I do an oil/filter change now or later? And should I just stick with the detergent in the synthetic oil? Or should I use synthetic with Kreen for the next OCI? Thanks for your recommendation.