Stupid question, I know. I'm just curious. If using a *-w30 through *-w50 in an engine meant for *--20w, and used over tens of thousands of miles, can you actually change the tolerances? I assume not but I assumed a lot before I got here!
Interesting question, since we know that fluid under pressure can cause erosion. I'd say it would take at least a 50 weight to do any significant damage. Even then, you would likely blow out seals before you eroded metal to any real degree, but an engineer could answer for sure.
I've seen the tin coating on bearings still there after tens of thousands of miles and that tin is very thin. And there are many engines that have had two or three top end overhauls and the mains and rods have not been touched, because they are not showing any wear. If changing to a thicker oil has gotton to the point of causing bearing wear, I think you'll see more than just a little wear. You would not be talking about wear, at this point, you'd be talking bearing failure. Bearings and engine manufacturing is becomming so precise that babbit, lead and copper are being replaced by aluminum bearings. Soon, a bottom end of an engine will never have to be touched for it's whole life, something that is already happening in many high quality engines, even to the cheapest rice rocket.
Dominic is right...OP must have meant clearances. Only way to increase clearances from using an engine is from engine wear. Permanent deformation from fluid (oil) pressures on metal parts is not going to happen. So the question boils down to does using a 30 through 50 weight oil in an engine that specs a 20 weight going to cause excess engine wear? **** if I know, but I sure wouldn't use anything thicker than a thin 30 weight in such engines.