THe only reson I don't use marvel mystery oil religiouslly, is because I have been pestered to use it non stop from my father. In all actuallity, its a minral based oil that provides exelent uper cylinder lube as well as dry ash lubrication properties (from burnt product) such as casteroil full synthetic provides). I use nothing but castrol 5-50. It sickens me that people do not understand the weight ratings on oil (thiers nothing that you can do beter for your engine that a good full synthetic). 5-50 means that the oil will not get thinner than a 50 weight oil at high temps (like what we get in texas), and will not get thicker than a 5 weight oil in cold (no chance of that here in texas, but plenty of it where I grew up in utah..... 5-50 is the broadest range of oil and I cannot believe everybody isn't using it.
Heads up everybody, your oil cap says 5-30 to satisfy the epa, reduce taxes for the manufacture and lower the buying price of your car. If you put 5-30 oil n your car, you are severlly misleed. Dont take my word for it, contact the oil manufactures, and ask them the benifits of 5-50.
The big point that I agree on with Quema34, is run full synthetic oil. Not only will you get better gas milage, you will get much less wear on your engine!!!
Unfortunatlly it is not good for old cars. If your car leeks a lot of oil, it will leek substantially more with synthetic....