low oil pressure at idle... smoke when starting car

i would get yourself a real gauge. the stocker sucks, and when the sender 'degrades' (degrades connotes a loss of performance or accuracy, so i say that with a smile), it is useless.
get a real gauge.
the STP (oil honey) should be ok. avoid stuff with teflon (which i dont think the STP you used has), and you are ok. im not a fan of stuff like that, though i have run it in oil cooled motorcycle engines (20W-50 and 105* does not work well with oil cooled bikes) and it helped pressure a bit.

i think oil puff at start up suggests valve seals are leaking a little (the oil drips into the chambers overnight and burns at start up). probably not a huge deal - i would be more concerned with oil pressure, via getting a real gauge (im not seeing that these two things are necessarily related). probably not a biggie.
just my two cents in the middle of the night.
good luck.

id vote for valve seals....