What could be the cause of oil in my radiator

(transmission ‘clutch’ material) While engine is off - ‘residual’ cooling system pressure will force coolant back-flow into the source of the lubricant contamination.. This will not result in complete lubricant depletion.. There simply is no room for ALL of the engine oil inside the radiator! Hope this is understandable .... In any event *hands-on* inspection / evaluation by qualified personnel will be required.