The picture posted by BLK98MK8LSC at:
Is an interesting observation, but not an uncommon phenomenon for just about any engine worked for extended periods under less than ideal conditions. Timing, mixture and simple heat buildup over time will all contribute to this phenomenon, more or less in literally any but the stoutest industrial designs. What you see there is simply a normal result of elevated EGT (exhaust gas temperature) and in no way indicative of what the oil may be exposed to in terms of it's temperature or temperature dissipation properties. It is heat affecting basically the piston crown, head (exhaust side) and exhaust tubing.
Assuming the engine's cooling system is operating correctly and loaded within it's capacity, that phenomenon has no affect on the oil, except possibly in turbocharged applications.
Cool pic, tho.