The chart clearly specifies either 5-30 or 10-30 synthetic for a wide range of temperatures. Either one will work just fine. I've noticed that the residue and discoloration that occurs in an unfiltered 4 stroke engine run on regular oil is noticably reduced with synthetic oil. The oil looks remarkably clean at the end of a season.

You should see what the manufacturer recommends to clean them. Could also affect any warranty issues.

If you mean that the system is steam, then the burner should be fired to match the load of the heating system, assuming that the boiler was sized that way. When downfiring a burner, the new firing rate should fall within the listed rate of the burner. If the air tube is an A, then you're at the low end of the firing rate. Quite often firing at the low end can be less efficient than firing more toward mid-range. When the nozzle size was changed was an efficiency test done to compare to the original? Also, you mentioned that the nozzle size was changed because the tech didn't have a 4.0 GPH nozzle. Was that one fouled? if there is good filtration there should be no need to replace the nozzle every year.

Cabots makes a line of outdoor semi-transparent stains that should work very well. I use their water based stain (redwood) and apply with a sprayer and back brush it into the surface for a uniform appearance. Be sure to remove all dirt/dust from the fence before applying - Good Luck!