my experience has been that the oil pressure sensor fails open (will not light the dash light) not closed. So I am thinking the sensor is good and that at Idle you have low pressure. I like to run 20-50 in the summer, but it's hard to find it up here in the north (canada) so I run 15-40. I don't know the history of your engine but the oil pump pick up screen can get cruddy, and the o ring on the pump can fail and lead to lower air pressure...also IPD sells a reinforcing ring for the oil pump...can't remember what for but it indicates a weakness there (this may only be for B18 though) Anyhooo, low oil pressure is a big deal, consider pulling the oil pan and taking a look, but first of all cahnge to a heavier weight oil and see what happens, if the light doesn't go on at low rpms after a hot day drive you are probably good to go, but if it does it's time to fix the oil pressure before you find yourself rebuilding the engine..
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Patrick, '68 220, '92 Eurovan , '53 PD4104 (highway coach conversion).