Your use of 20-50 weight dino oils if within parameters of your owners manual is ok. Synthetics, on the other hand, have more stable viscosity at higher engine operating temperatures. Therefore, a quality synthetic 40W at high engine temps provides better lubrication than a dino 50W. See the redline web site for a good discussion of synthetic vs dino oil. http://www.redlineoil.com/products.htm
A partial discussion follows:
VISCOSITY GRADE SELECTION
The proper viscosity grade to use depends on many factors such as ambient temperature, miles on the engine, bearing and piston clearances, and type of service. A general rule of lubrication is to use the lowest viscosity possible which will provide the proper separation of metal. Anything more than this minimum will increase power loss due to friction and will reduce the pumpability of the oil at all temperatures. However, one must consider synthetics differently when determining viscosity requirements. At high speeds, a petroleum 5W30 can appear to be a 10W in the bearings due to the shear stress, yet Red Line 5W30 will actually be an SAE 30 in the bearings. There is a significant difference in lubricating an engine with a 30 compared to a 10W. Red Line Synthetic oil will provide better viscosity protection than the next higher petroleum viscosity grade. If clearances are tight and very little lugging occurs, then the 5W30 or 10W30 should be adequate. Less turbo lag will be noticed with the 5W30. If ambient temperatures will regularly climb above 100°F, then the 10W40 would provide an additional safety margin. If the engine is air-cooled, or if the engine is older and has greater bearing and ring clearances, the 15W50 and 20W50 will develop a thicker oil film. If very low temperatures occur, the lower viscosity grades, (5W30, 10W30, or 10W40) will flow better and lubricate the engine more quickly after start-up.