Multi-weight or multi-grade thickens as it warms; 20W50 starts at 20W, for example, and increases to 50W as the temperature increases; at operating temperatures, it performs with the properties consistent with 50W oil.

Many years ago, it was common to use fuel injection to the oil tank or crank case to thin the oil with fuel at or just after shutdown.

Some oils have become difficult to get. About the only radial oil available now is Aeroshell 120, and it needs ordered by the barrel (which is fine, because it's not uncommon to take 3-5 gallons of oil per engine per hour on a large radial). In the past, when working around the country, we had to order the oil in advance and have it shipped to the area we'd be working, because there was near zero chance anyone would have it. I usually carried a couple of drums aboard, too.