OK, here's a short tutorial on oil, liberally spiced with my own prejudices on the subject. Getting old and opinionated does that!
"Detergent" oil was developed in the early 1950's, and was a very controversial subject at the time. It came about by adding a series of chemicals to basic engine oil that would suspend, and disperse tiny particles of dirt too small to be trapped by conventional oil filters. When the oil was changed, the dirt was drained out with it. Older engines which had run without filters, especially, would start to consume oil after being switched to detergent oil. The dirt built up behind piston rings, etc., was the only thing keeping them in contact with the cylinder walls. It's highly unlikely that any engine that's survived the past 30 years or more has been run without at least a little detergent-type oil. so using it won't suddenly start one burning oil. If that happens, the engine's worn out anyway!
"Weight" is the viscosity of the oil, how fast it can flow through a calibrated-size hole in a special measuring cup. Straight-weight oils, like 20, 30, 40, 50, etc., stay the same thickness all the time, only thinning slightly as they heat up to normal running temperature. A "Winter" rated oil is flow-measured at 0 degrees Farinheit, and has a "W" in its designation. For instance, "SAE 30" oil is measured at summer tempertatures, 80 degrees F., I thing, while "20W" is a winter-rated oil, flow-measured at Zero degrees. Now, for multi-viscosity oils- - - - - -
10W-30 oil flows like a 10 weight at zero degrees, and lubricates like a 30 weight when it's hot. It DOES NOT change thickness! It remains thin, like a 10 weight, regardless of temperature. A 10W30 oil in an engine with lots of clearance won't cushion the parts like a 30, and will allow it to rattle, and burn oil because the rings can't seal the thin oil as well. It can lubricate a hot engine like a 30, but it's just not thick enough. 20W50 is a pretty good compromise. It's not quite as thick as a straight 30 oil, but it lubes like a 50 when it's hot. It also flows like a 20 in cold start situations. "30 HD" means 30 weight, high detergent oil. It has the viscosity of 30 weight oil all the time, but it has the detergent-dispersant additives. It thickens up like molasses when the weather gets cold, putting extra load on the battery and starter, and robbing the engine of lubrication for a time on a cold start. The easiest place to find a 30 HD oil is a truck stop, as a lot of heavy-duty Diesel engines still run it. It won't hurt to run a Diesel-rated oil in an old Stovebolt engine.
Jerry
