I'm simply gonna have to call Merc out on the "it OK for one side to be hotter than the other". Engines are all about balance, if 4 cylinders are hotter than the other four (and I'd say 20 degrees is getting to be a BIG difference) then you are pushing one side harder than the other. Eventually rings/lifters/valves/your cam/crank will all wear due to the imbalance. Something is a miss. Could be a bad ring, bad valves, clogged manifold but something is bad. I would check the exhaust. Look for any smoke on starting it up, see if one side smells richer than the other. But, bottom line is, if it runs with no problem but the heat then is it a Big deal? I'd say run it until you see the ill effects then the problem will be easier to identify but no more expensive to fix, hopefully
Good luck, and happy waken