When to worry about engine noises?

SOME (NOT ALL) Synthetic oils, find leaks where nothing else will. At a molecular level, it's man made, and particles are the same size/shape. Like turning an egg timer full of fine sand over. It'll all find its way down the hole eventually. Picture the same egg timer with coarse, non uniform sand. Doesn't drain as fast....may even stop "leaking" entirely. The extreme slickness of synthetics is what makes them problematic on old school engines. My opinion. And having seen cross hatch "polished" right off cylinder walls, that's just my stance. I love synthetic in my hemi's. But on my old school engines I drive all summer and change once a year....why pay $15 a quart for some fancy oil designed for a 20k mile interval, when the oe specs of these dinosaurs was a 30 wt old school oil. My 2 cents. I'm not a scientist, but some pretty smart guys have pointed out the above to me.