"Lack of oil" does not necessarily mean "no oil" or "too little oil" in the engine. It means there was a lack of oil where it was needed, and that is the case here. For some reason, the journal did not get enough oil for a period long enough to START the problem. After the problem starts, it is all down hill from there. The bearing spun because it fused to the crank journal for a split second. It fused to the crank because of the heat from insufficient oiling.
Excessive vibration could be an indicator of a worn crank journal.
Some (not every) possible causes of Lack of oil:
Change from one oil to another and they don't mix well.
Oil viscosity too thick during operation
Engine sits up for a long time period with low viscosity oil then run hard too quickly aka "dry start"
Fuel contamination of the oil. (your picture shows severe cylinder wear)
Water in the engine from washing.
If an engine tightens up, boggs down, or suddenly shakes more than normal, TURN IT OFF IMMEDIATELY! You may lose the race, but you will save your engine block.
Plain bearings, when properly clearanced and lubricated never touch the crank journal. There is a hydraulic wave of oil that is forced into the tight point between the bearing and crank during motion. The only time the crank and bearing will normally touch is breifly during startup. If they touch even the slightest bit during operation, all heck breaks loose in a big hurry. In a splash oiling system (aka "oil by chance") you don't always get oil where it is needed fast enough.
This is why there are multi-grade oils. 5w30 is a 5 weight at cold temp, and 30 weight at operating temp. With these engines, I would run a 5w20 and give the engine time to warm up before racing. In colder climates maybe even run a 0w15. In hot weather (90° and up) you may even go to a 5w30. Always use a pure synthetic. Don't be scared of thin oil as long as it has good friction modifiers and film strength but i can gaurantee you that ANY oil which is too thick will starve your rod journal and give you premature issues...sometimes catastrophic.
Never use an oil designed for methanol in an engine running gasoline, and visa-versa.