The problem is, that a 5W40 is thicker when hot, than a 0W40.

It seems obvious if u go off the above facts, that we should all be using 0W40. Indeed, 0W is the best grade for any engine when it's cold - as its almost impossible to have an oil too thin on startup.

I have done a lot of tests with diff oils measuring temp against pressure. A CVH should run 40psi of pressure when oil is hot, at 3000rpm. My own engine has covered less than 10k since a very thorough rebuild. It does however, have quite large bearing/crank clearances as common with race engines - but these are still within tolerance for a new engine.

With Mobil 0W40 - i can get no more than 32psi at any rpm. With Valvoline racing 5W50 i get around 38psi, and with millers 10W60 i get 40psi. My concern with millers is the cold start protection, but being an ester synthetic it should be good.

It should be noted, that pressure doesnt lubricate, flow does. However, 32psi is a little low to prevent crank/bearing contact in extreme conditions. My aim is to find the thinest oil which give the correct 40psi/3000rpm at 200F. Going to try some silkolene next.