if the inside of the engine oil cap is milky, it indicates there is ***** or coolant in the engine oil. It may be a head gasket leak. ***** or coolant will eventually break down the engine oil and the engine will seize and you will be replacing it. Average cost to repair a head gasket is $250 - $500.
If the inside of the engine oil cap is sticky. there may be STP or another engine additive in the the oil. STP and additives are thick and designed to quiet engine noise. It is there to mask a problem. Average cost for engine replacement: $1500 - $3500.
here are some more tips on how to avoid these
Dealer Tricks
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