Only when idling in DRIVE like at a stop light. Doesn't come on when idling in PARK or NEUTRAL. Oil Pump replaced not that long ago. Any ideas?

On an 81 we bought recently the oil light glimmered when I came up to a stop sign and put in the clutch. I don't know what kind of oil was in it, but when I changed it to 10-40, the problem disappeared.
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Thanks everyone for all the information and advice, Doug C. 81 242 Brick Off Blocks, stock, B21F (non-Turbo), M46; 86 244, B230, 140k , auto.

depends on the milage. in drive with the brakes on the engine idles lower than in park or neutral. My 240 with 350,000 km flickers at idle if I use 10W 40 and doesn,t if I use 20W50. Low oil pressure is one of the symptoms of and high milage engine. Or like the other guy said it might be the sender. no big either way as the engine needs pressure when working more than at idle.

Sending unit is messed up.. The lower RPM's in drive or reverse can cause the sensor to read a low oil situtation. Check your RPM's and see if it might be a little low but more than likly it's the sender unit. Put a manuel gauge on it and check the pressure at idle and above idle also to see if the presure rises in conjunction with the engine rpm's. not sure about the reading for idle pressure but I would think anything around 35 lbs. should be fine at idle. Call the dealer and ask the service department. They should know. Someone here will surely have the readings..

I recently solved this problem by putting in a new oil pressure sensor. Cost $10 from FCPGroton, was not a Volvo sensor, and the flickering oil light is gone (oil pressure is fine - had it tested). I also noticed that my sensor was only finger tight!!! Can't believe it. Leaking oil as a result. Now all better.
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Darius in Albany NY '86 744 GLE (175K, given to my mother-in-law); '91 745 Turbo (174K-my 'Little Red Wagon'), '00 Toyota Camry (65K - my wife's transportation appliance)