switching between non-synthetic, or more technically "castor" oils, is no problem at all...there's no reason not to switch between them, its not going to hurt your car, synthetic oils tend to maintain a constant viscosity, meaning better flow, which also means less lug on your engine, which means more mpg. i've seen cases of 2-3mpg gained just by switching motor oils. and i've also seen gains of 10hp, and this is RWHP by switching gear oils in manual tranny's and also differential gear oil changes. synthetic oils have a higher evaporation point as well, meaning that they wont disappear due to oil burnoff as quickly from a hot engine, however, if you hve a leak around a piston skirt or ring, then it's going to burn faster, its just one of those things. overall, synthetic is better, but who needs it? nobody, unless you're super crazy on your ride. if you redline everytime you take off from a stop light, then you could make a case i guess....but its 3-4 times more expensive, and i say not worth it unless you're in lemans.