3 Answers

....opinion: no

I am not aware of anything to the contrary. While elements can be harmless all by themselves, e.g. NA  (sodium) and OH ( hydroxide) combining them will produce lye, which can kill if ingested, I can't see that being the case with oil. If anything, combining the 2 in a vehicle would enable it to flow more easily and be less subject to extreme temperature conditions. I live in NY, and park my car outside, where there winter temperatures can be brutal. Therefore, I use synthetic oil exclusively. Yes it's expensive relative to conventional motor oil, but the oil doesn't have to be changed as often. The only viable reasons I can see for mixing them, is the unavailability of sufficient quantities of either, or it's the cost.

I once had a jeep cherokee sports,and under no conditions was i to put anything but synthetic oil in it..>>>>>>>><<<<<<<<..