My theory is full of "any" lube ,is far better than running it low or empty!..even used lube is better than running something too low or with none..I wouldn't worry aboyt mixing viscosities as much as running it too low..just change it ASAP when you can just in case...
Case in point,my 6.2 has been leaking a quart a day out of a rotted oil pan I cant seal up 100%,until I get the pan replaced soon, I've resorted to using drain oil from a friends shop,filtered well of course--I know its not "right" and could damage the engine--but not as badly or as quickly as letting it run OUT of oil,or with just a few quarts in it,instead of all 7 qts..with 15W-40 diesel rated oil going for 5 bucks a qt, or 15 a gallon,I dont have the cash to fill it with 'new" oil daily,only to have it leak out and wasted..
My biggest fear is there might be other fluids like brake fluid,ATF or gear oil mixed in with the "used" oil I've been using--but he's been pretty good about catching the oil in a clean 5 qt, container for me,right out of the engine he's draining,not just using his roll around drain oil "collector",which then gets emptied into 55 gallon drums--then "anything" else petroleum based might get dumped in there with it..
Much of the oil I get looks "clean"he has several customers who get oil changes every 2000 miles on high end cars (and some might have synthetic oil)--,but I still filter it well by letting it drain through a funnel lined with cotton t-shirt material and nylon stocking mesh ,and I dont use the "last" of it at the bottom of the container..I hope to get my oil pan changed in the next week,so I can go back to 15W-40 and pray no damage has been done --so far the engine sounds quieter than it ever did,and spins over faster when starting it with the thinner oil..
I once drove my '72 K5 back from NH about 110 miles once with SAE 30 and a pint of STP in the rear diff...it had a rusted diff cover ,and a large scab flaked off it while I was up there on vacation--on my way home I noticed a trail of oil behind the truck as I was filling the gas tank--it was a sunday,the gas station was a self-service one with no mechanics or bay,and the thickest lube they had for sale at the "shack" was SAE 30 motor oil,and some STP oil treatment..
I knotted up and poked a plastic trash bag into the hole in the diff cover to hopefully cork it up ,and filled the diff ,luckily I had a funnel and a hunk of fuel line with me,and the thing didn't leak bad,just weeped a little...I was paranoid all the way home my diff bearings or gears would get hot and fail at highway speeds,I kept it under 55 mph --I did make it home OK,and bought a chromed diff cover the next day and some 85W-140 gear oil and I never heard any protests from the diff afterwards..
