When I was doing research in grad school I found several instances of infantrymen using diesel oil to lube their M16's in Vietnam during the years when the government was not keen on issuing cleaning and maintenance supplies for them.
Use of engine oil on your M-16 was even discussed in the Army FM... a bit from the dipstick was suggested. The key point in this case was: M-16s run better with lube. Any lube beats running dry. Even used diesel oil from a truck.
But that doesn't mean it's preferred over dedicated formulations.
As mentioned earlier : Pneumatic Air Tool Oil (Lucas) stocked by Advance Auto ... Clean , no smell, great gun lube . Also M1 20W50 V-Twin Synthetic oil (hi molly content - no smell) and lastly , Dexron VI ATF (almost all synthetic - no smell ... very slick) .