The are huge frictional losses are in the valve train. In fact flaying surfaces between the cam followers and cams lobes are subjected to such huge loads that both components must be made from very hard high alloy metal, even though this flies in the face of good engineering practice.
Certain synthetic lubricants and additives can achieve extremely high film strengths (>150,000 psi) and in pre-ECU vehicles, I have personally seen the idle speed increase by as much as 250 rpm without touching the idle adjustment.
At higher rpm, shear plays a big part too and synthetics again, tend to be better in this regard.
Noone in F1 is ever going to run API SJ 20W-50 in their $50K engine but if they did, it would not surprise me if they lost 100 hp.
Don't take this post as a plug for synthetic lubricants generally. I can tell you from testing a lot of them, that many are quite ordinary at fulfilling their primary function, that of keeping pieces of metal apart...