keep in mind that the molecular structure or synthetic oil is much smaller than natural fossil oil. While this is a plus because it can better lubricate the engine, if your piston rings let a little regular oil by between oil changes, they're going to let a helluva lot of synthetic by if you switch. Another thing to be wary of is if the engine is old, and even though your seals are still good at keeping the oil in the engine and keep it from seeping out of nooks and crannies, synthetic may find its way out, making a mess, and making you think your head gasket just bought it. While I know this, I myself am pondering whether I should/can start putting syn in my car. It's a 94, so about 9 years old, but only has 54k miles on it... I need to consult with an expert.