I want to know at what percentage of slope, engine oil touches oil pan gasket material? Is it harmful for the gasket?
A 40% slope is well beyond what you will find on nearly any actual street in the world. But assuming you did find such a place to park and if your oil pan was designed such that such a slope would put the oil in contact with the gasket, would that be harmful?
Well, considering that under normal operating conditions there is nearly always oil in contact with the oil pan gasket I'd say that the answer is no, that would not be harmful to a normal (i.e. uncompromised) gasket.