Does a cool head protect oil viscosity?

My last post got me thinking. Cylinder head temperature can be very high even if engine oil temperature is moderate.

That means the engine oil does not do a very good job of cooling the cylinder head. If you have a look at a Harley oil flow diagram you can see why. Only a small amount of oil travels up through the push rods to the top end to lubricate rockers etc. The oil flow is not designed to cool.

But while that small amount of oil is flowing around the top end, it may be getting super heated. It may be picking up head temperatures of close to 300°C.

We know that high temperatures degrade oil. In fact, lubrication capacity decreases rapidly over 120°C. (see the info on Cooler engine oil)

So, oil may be overheated and damaged as it moves around the head, and then return to the sump where it cools – but the damage is not fixed up. Over time, all the oil in the engine will be affected. Maybe that’s one reason why Harleys need frequent oil changes.

If I’m correct, keeping the cylinder head a little bit cooler by using engine cooling fans in traffic could be very beneficial.