Recently I have been noticing a smell of burnt oil inside the cabin of the car. This is sporadic and I notice this after driving the car for about a half hour with the heat on. I had the oil changed at 3700 miles, now at 6100 miles. I have checked oil level and oil color with no problematic observations. I have pulled the plastic engine cover and examined the block and have not seen any oil on the surface. Any thoughts or similar issues?