I don't remember the specifics, but do remember reading at some point that synthetic oil responds differently and can actually cause a lower oil pressure at lower RPMs because it is less viscious but does a better job lubricating
Switching from conventional to synthetic will not affect the pressure at all, provided you stay with the same weight. If you go from say a 10W-30 to a 5W-30 you MIGHT notice about a 1psi difference in hot pressure after a long highway run. For the most part though, that minor of a change you'll see no difference. This is the same no matter what type you run, conventional, full synthetic, or blend.
Now if you go from a 10W-30 to say a 0W-30 (not recommended unless you've got a diesel) then yes, you should see a noticeable difference in pressure readings, and it will be lower. The opposite of course if you go to a higher weight, like a 15W-whatever (again not recommended..especially this time of year).
My guess, you've got a bad sender unit. You're seeing the light come on once it's got up to temp and the oil has naturally thinned out a little bit. If it's knocking and making all kinds of bad noises, then you've got a problem. From what i'm seeing though, the sender units tend to go out on the KJ for some reason. Pretty cheap and easy fix.
