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"Mayo" on the oil filler cap can be caused by just doing short trips and never letting the engine warm up properly. A 50 - 70 mile trip will cure this if it is. However, if it is more serious, a 50 / 70 mile trip will not be good for the engine at all.

However, (there's always a "however") if it has come on recently and your driving habits have not changed, it may also be a sign of a leaking head gasket or a cracked cylinder head. The "Mayo" is an emulsion of oil and water. Once it gets to this state, the lubrication and heat dispersion properties of the oil/water mixture go downhill.

You may experience lumpy running and/or white smoke from the exhaust and/or a noticeable loss of coolant. Theoretically, higher revving could be caused by "water injection"; the induction stroke of the piston draws in water from the cooling system, the ignition of the mixture in the cylinder then turns the water to steam, which provides a little more power, and hence the slightly higher revving.

If the head / head gasket has gone, the compression on a cylinder affected by this will be slightly lower because, when the piston rises on the compression stroke, some of the volume escapes into the cooling system. If it's just condensation from short trips, the compression will be stable across the cylinders.

Solution, get the compression tested on the cylinders and a pressure test on the cooling system - it shouldn't cost that much.
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