oil pressure drop

2002 S10 4.3 with about 38k on it.
Noticed last night that oil pressure gauge was reading lower than normal.
Midscale is 275 kpa (39 PSI) and it normally runs about there or higher when
hot at highway speed Last night it was down about 1/4 scale at speed which
it normally only would go when at idle stopped. Crawled under and noticed
small amount of oil on the filter but no dripping.
Difficult to check oil level when hot and with flashlight so added about 3/4
quart and restarted. No real change, so left it until morning. Checked oil
level and it was now actually a bit above max on dip stick so wasn't really
short of oil. Started it up and it read 2 bars above midpoint ( there are 4
bars per quarter of gauge range which is pretty normal. Took short drive to
store and back. By the time I was back home the reading had gone low again.
Gauge needle moved more with rev change than last night but still seemed to
go to 1/4 point above idle. Read somewhere that senders can go and oil will
be in plug. Wonder if that might be the source of oil on filter, assuming
sender is near filter base. Is this a common problem on these motors? Could
I be losing pressure hot or maybe sender is now measuring wrong when hot.