Always swore by synthetic oil until I saw the carbon buildup firsthand

I used to think synthetic oil was the best thing for every engine, no questions asked. Then I tore down a 2015 Honda Accord last month with 80k miles that only ever had synthetic. The piston rings were caked with carbon way worse than any conventional oil engine I've seen from that era. A senior mechanic I work with pointed out how some synthetics don't burn off as clean in direct injection engines. Now I'm recommending a switch to high-mileage conventional for certain cars, at least for 5k intervals. Has anyone else noticed this on DI engines or am I just seeing things?

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max_jones4mo ago

Man I saw the exact same thing last year on a buddy's 2017 Mazda 3 with about 70k. He only used Mobil 1 synthetic since new and when we pulled the intake valves they looked like someone spray painted them black. Carbon was so thick we had to walnut blast just to get the engine running smooth again. My old 2005 Camry with 180k on conventional oil had cleaner valves than that Mazda. I think some of these newer synthetics are so slippery they don't burn off right in DI engines, they just leave behind this nasty crust. Now I tell people to run a can of Seafoam through the intake every 10k miles or so if they're stuck with synthetic. Its not perfect but it sure beats pulling the head off at 80k miles.

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gray_nelson4mo ago

Took the valve cover off my buddy's Genesis and found what looked like a charcoal briquette factory in there.

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