i've always German torque, good&tight. tight enough that i don't want them to fall out, not tight enough to break anything, and always use a proper spark plug socket with some kind of rubber protector inside it to prevent cracking the plugs.
here's a page that goes over it, i'm sure there are many more for looking
they actually make a specific torque wrench specifically for spark plugs! if you're not familiar, it's not hard to reach 20 ft-lbs with a normal 3/8 drive ratchet, no gorilla tools needed. most guys use a dab of anti-seize on the threads to prevent galling on the next service.
how / why would an EFI system run rich?
if the 02 is reading lean due to an exhaust leak and keeps telling the injectors the engine needs more fuel?
or the 02(s) is past it's prime or fouled and not sending good info