Hi guys. Car Care Nut said by nature of internal combustion engines, fuel will leak into the oil, which is why he recommends not following the manufacturer when they tell you to wait 10,000 miles for an oil change. I had no idea small amounts of fuel were getting into the engine oil. Is it true?
Is it true?
Of course it's true - and 10,000 oil changes are wrong for a lot of reasons. Change oil and filter at 5000 miles or one year, whichever comes first.
@chucktobias of course? Is it something obvious? You care to give details or elaborate?
Rings are not a perfect seal.
You're referring to a phenomenon called blow-by. Fuel doesn't deliberately leak into the motor oil. It really depends on the type of engine you have, how far you drive, and the stiffness of your piston rings.
My 1999 Ranger and my 2017 Mustang both get to operating temperature before I stop the engine. Blow-by from the combustion process feeds back into the vehicle's PCV valve. When the engine gets hot, the gas is quite volatile and the engine will release the blow-by gasoline from the oil in the engine's PCV valve.
My 2017 Mustang and 1999 Ranger have ordinary fuel injection, which injects fuel into the intake manifold at 60 PSI. Direct Injection, on the other hand, directly injects fuel into the combustion chambers without flowing over the intake valves first. Idling on a GDI engine means you're putting fuel that's around 200 PSI directly into the engine's combustion chambers. As your engine revvs higher, the PSI increases. The more pressure there is in the fuel injection process, the more blow-by is going leak past the piston rings and cause oil dilution.
GDI and turbocharging is a massive double whammy.
On the other hand, cars like my old Pontiac Catalina feature carburetors. The engine is powered by the relative vacuum in the engine as the piston moves down during the intake stroke; carburetors are not pressurized. The piston rings are also really stiff; I run 10W-40 in that car, and it's compatible with 20W-50 depending on the ambient temperature at the time of starting. Comparatively little blow-by happens with the Pontiac, but it still happens, and there's the same emissions control hardware.
20W-50 is a heavy oil. Ultra-modern engines require 0W-16, which reflects the massive decrease in piston ring tension. Reduced piston ring tension improves gas mileage, but comes at the cost of at-times massive blow-by.
That's the nuts-and-bolts perspective.
Change your oil at least once a year or 5,000 miles, and your engine will be happy.
