Any gearheads know if it's better to burn old (as in many months old) gas & then refuel, or gas up with the old gas still in the tank & hope whatever solvents are in the fresh gas ameliorate the condition of the old gas?
This might be a good question for
@RWD, but my take is:
If we're talking months and not years, and if we're talking gasoline (as opposed to diesel), I would say - just fill up and go if you're planning to use it within a reasonable timeframe. If it's more of a constant condition of almost never using then consider a fuel stabilizer. If you really don't trust the gas in the tank, then empty the tank in another way than driving and discard responsibly of the old gas.
A couple of perhaps relevant examples from my neck of the woods:
My only car is a "summer only" car that I park at end of October each year, and start again in the beginning of April. I always fill up the tank before this winter break and I've never had any problems with the fuel. A full tank reduces the risk of water consdensation when sitting still - at least that's my thinking. (Gasoline, not diesel).
A friend has an old enthusiast summer car (Triumph Spitfire from 1972) that is normally parked during the winters. One summer it never got going because of other repairs and when he finally got it on the road after sixteen months of sitting still with a full tank of regular gas it still was starting and running OK but fuel consumption was significantly higher (30-50%) for the first tank. Once he got fresh gas in the car it went down to normal consumption. (Again, gasoline, not diesel)
And... on an old family farm there was an old diesel tractor used for hauling lumber in the forest. Once that tractor sat for a few years before a family member tried to start it with the (at least) three year old diesel fuel in it. Modern diesel fuel in my neck of the wood is not that stable, to many bio-additives to use in that way. It turns into some sticky jello-like substance so after that start attempt we had to empty the tank and also clean out all fuel lines in the engine etc... but after cleaning and with fresh diesel it started fine again. This example is perhaps not applicable for you - neither the same diesel where you live, nor as long time sitting idle I hope. This is more of a lesson for those who have e.g. a diesel fueled power generator for emergency use... don't use regular truck diesel if you live in a place like I do. Buy the good, stable stuff even if it's more expensive.