I don't want to start an oil thread about the pluses or minuses about synthetic/semi-synthetic oils. I would like to know simply why "semi-synthetic" oil exists. We all understand regular motor oil and synthetic oil have their definite advantages, but why would you use a half-and-half combination? I know it's cheaper than full synthetic, but isn't that false economy? It's almost like brushing your teeth without toothpaste, or changing your oil without changing the filter - you're only completing a partially effective action. I noticed that Saab routinely uses (and I'm sure others do too) "semi-synthetic" oil for their late model cars. It just doesn't make sense to me.