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the main problem I have with using engine oil (and yes we used to actually use engine oil from our cars) but anycase engine oil doesn't stick to the chain for very long and can allow the chain to dry out, and get hot as well as wearing the front of hte bar faster dulling the chain faster, not cutting as well put more wear on the drive sprocket etc. Personally I have long since learned to stick with bar, and chain lube.
You can use 30 weight motor oil (this is coming from any Stihl owner's manual) in a pinch, but they suggest you use bar and chain oil
Bar and chain oil sticks to the bar and chain better and provides better lubrication.
However, if you run out of B&C oil, you CAN use 30w motor oil.
Octane
Cutting firewood is not going to put that much wear and tear on the saw, so don't waste the extra trip. I have used everything from 5w-30 to 20w-50 in my saws to no ill effect when I ran out of bar oil including milling. Once the saw is running for a few minutes, the oil tank will become very hot and oil weight will matter little unless it is -10 outside.