I recently had the engine rebuilt for my Merkur XR4Ti. It is a Ford 2.3L Turbo SOHC. The engine was built with factory specifications on the bearing clearances. The cam is a hydraulic slider cam, not roller followers. I have approximately 3000 miles on the engine at this point. I have changed the oil a couple times. The break-in was with Autozone 10w30 and Comp-cams break-in lube. The second oil change after about 10 miles was the same. At 500 I switched to Castrol GTX Diesel 15w40 and at 2000 miles I switched to Valvoline All Fleet Plus 15w40. I've been using the deisel and all fleet because they are few non-SM oils that I have been able to locate. I'd like to switch to a synthetic, but I figure I'll need something with high ZDDP levels because of the slider cam. This is a daily driven car, but I will be taking it to open track driving events as well and I'm sure the oil temperatures will get quite hot. I'm looking for something that is available off the shelf. I have Napa, Autozone, Kragen, Pep Boys, and Walmart near by. The owners manual recommended API SF 10w30 or 10w40 for 0°F+, 15w40 for 10°F+ and 20w40 or 20w50 for 20°F+.