here is a trick for you, next time you change your oil cut the can off of
your filter with a hack saw and check the pleats for metal particles.  i
would guess that running oil pressure this low in a 1.9 is going to start
the bearing eating process very soon if it hasn't already.  most of the
1.9 cases i've taken apart were exhibiting movement of the main bearing
pins.  if you are looking to ever rebuild this engine or get a decent core
value out of it i would suggest not running it until it spins a bearing.

if you are planning on just running it into the ground you could do stuff
like run motor honey with your 20-50, but its only going to do so much.

jonathan