The gas pressure is actually low, it should be 9"-11" WC.
As you know, backfiring is typically indicative of a lean fuel mixture or timing issue.
The low oil could be due to worn valve guides, or possibly a bad valve seal.
If this set is used for standby only, there is not reason the valves should need to be adjusted every year - although it certainly wouldn't hurt anything.
I suspect that most of your issues are going to be due to the low gas pressure - this would obviously also cause hard starting.
I would definitely adjust the gas pressure, and if it has been quite a while, adjust the valves.
I would think this will take care of it, but if for some reason it does not, let me know.