there is various theories whether its bad or not to run the tank nearly dry etc etc
personally i believe that there shouldnt be much buildup in the bottom of the tank, and i always run the tank on my excel empty with no side effects
try a few times running the tank "emptier" so to nearly on empty and see what happens
if it seems to get rough and surging and lurching, then maybe its time to replace the pump (as teh filter sock is prob stuffed) and also check the fuel filter aswell
re: oils and the like - ive seen the results of changing all the oils to the Royal Purple brand (motor oil, diff oil and gearbox oil), with a power increase of around 10rwkw normal as this is a full synthetic, quality 0w (thin) oil
depending on the climate, a 0w oil might be too thin, but a good quality 10w or 15w semi or full synthetic would be the goods (and its not like a turbo motor oil, where the oil kinda gets 'heat cycled' by the turbo and supposedly breaks down quicker and therefore needs to be good quality and changed often)