My Falcon sat for almost a year without having the engine run to keep everything lubed and clean. When I went to check the oil level, there was sludge on the edges of the dipstick, and the oil was black and thick. In addition, the rear main seal had dried up and oil was seeping out from other gasket areas as well (all around the oil pan, for example). Since I was down almost 2 quarts, I dumped in a quart of ATF and topped it off with 10W30. I ran the engine for a while until the valve clatter quieted down and the engine was good and warm. Then I let the car sit for a few days before dumping the oil and refilling with Castrol Hi-Mileage oil (regular dino oil with some seal conditioners). Everything has been A-OK since then.
Would I do this in my SHO or my SLO? Nope. But it's fine for a $30 motor made to 1964 standards.