[Destroyer]

Viscosity is the resistance of a fluid to flow or deformation. In everyday terms (and for fluids only), we can say that viscosity is "thickness" or "internal friction". So a lower viscosity oil will provide less friction (friction in this case being lubrication quality) than a higher weight oil. So the less friction there is, the easier it is for parts to rub together producing wear. Actually, 5w20 would probably work fine in your engine in the winter time when you want a little thinner viscosity index, but for a high milage engine like yours you're better off with a 10w30 or even a 10w40 for the summer when the additional heat tends to thin out an oil. But remember that with all the additives and different blends in oil these days the oil itself is not actually 5 weight or 40 weight thickness... rather, it gives your engine the lubricating qualitys of a 5 weight or 40 weight or whatever it is that you happen to put into the crankcase. In all cases, good catch on the oil.