Nasty one PJ. Agree with brian, not an oil problem - an ignition/valve/fuel one. Is it the exhaust valve side that's been eaten right through to the rings? Looks like terrible detonation, but I'm sure it would have been rattling away like anything, as Brian says, to have done that much damage - twice!. Assume the plugs are a) both the same and b) correct for the engine. What do they look like I wonder?
With those pistons, octane rating shouldn't be a real problem I wouldn't have thought unless your fuel supply is adulterated. Number 1 thought is also (a quite large!) timing variation between cylinders, perhaps compounded by weak mixture (induction bias?) and less than perfect valve or seat. What do the underside of the head and the valve heads look like now? Are there great big lumps of hard-as-concrete stuff attached in various places causing pre-ignition through being incandescent all the time? And the inlet and exhaust tracts in the head and the backs of the valve heads? Are they all reasonably clean?
Whatever, it's not good or pretty, but it'll be one of these things or a combination - best of luck.