How do hydraulic flat tappet lifters fail?

I hate to beat a dead horse, but I was reading an oil viscosity chart, and I think I may be running the wrong oil. This chart gives suggestions according to bearing clearances, and my main bearing clearances were .002". According to the chart, I should run 20 weight or 30 weight. I was running 40 weight. Would that cause an oiling problem at higher RPMs or higher temps?
No not like that, not that fast! The lifter didn't rotate for whatever reason. That probably started day one.