At 15K miles the used oil analysis showed 0.5(%?) fuel in the oil, while previous tests all showed <0.5. Flash piont also went from 395 to 355, presumably due to the fuel. Is this something to be concerned with? The driving pattern did change. Previously it was about half local and half highway. For the last 5K miles it has been almost all local so the ICE would be on, off, on, off.... unlike on the highway it would be always on. Am I hurting the engine by not driving on the highway frequently?
No 0.5% isn't really anything to be too concerned about. A little bit of fuel always gets past the rings on cold start and then it gets driven off as vapor (and recycled through the PCV system) once the oil is fully up to temperature. So the fuel dilution is not necessarily a cumulative thing. Even just one longer trip with the engine nice and warm would normally be enough to send it back to <0.5%.
My UOA reports have always had a bit of fuel in the oil even at the 7500mi OCI that I use. Given that the engine is always stopping and starting, and that when it runs it tends to run at very high load, I think it makes sense.
0.5% is OK and no action required on your part. As others have said, it would be lower with more highway driving.
You can minimize the amount of fuel in the oil by ensuring the engine gets up to a reasonable temperature when driving in the city. You'll need a scangauge to watch the coolant temperature. Once you have one of these, you can block the lower grill to allow the engine to warm up more. With both grills open, in the city in fall/winter, you may notice the coolant rarely gets to the thermostat opening temp. When you force it to warm up as above, the fuel will vapourize and be burned via the PCV system. This will lower the % in the oil, but there will always be some in there. Too much fuel in the oil (I doubt there is a definitive number for this - it is likely a more is worse, less is better situation) will cause excess wear.
I do have ScanGauge and do block grills partially even during summer. The max coolant temp during each trip is in the low 190s(F) - well above the thermostat opening temp but maybe not up there long enough for the fuel to burn away. It seems to burn it away it needs highway driving where the ICE is running continuously. The wear numbers came back fine from the oil analysis.
On warm-up oil temperature lags coolant temperature by a good 10-15 minutes, sometimes more. It varies by car and driving cycle.
