I think there are differences in conventional motor oils and some may be better than others. But I think you could take any major brand conventional motor oil that meets all of the current requirements (Pennzoil, Quaker State, Castrol, Valvoline, Chevron, etc.) and that motor oil would do just fine in 3000 mile or three month oil changes.
And motor oils today certainly are better than even a few years ago. Sludge in an engine today is probably the result of few or no oil changes, using a motor oil that does not meet requirements, etc.
And even if a typical conventional motor oil might cause some sludge, cleaning the car/truck engine with Auto-RX at 25,000 mile intervals would probably take care of the problem.