Oil pressure light coming on at idle

Some of the senders are a little more alarmist than others and will tend to trigger the light at the low end of normal oil pressure, especially with hot (thin) oil and slow idle in gear. Take note that when the oil pressure sender diaphragm starts to leak, you get unwarranted oil light illumination and a tough-to-find oil leak. The proper thing to do would be to put a mechanical oil pressure gauge on the engine and check to make sure you've got adequate oil pressure, but if you haven't got one readily to hand, it may not be worth your hassle to mess with a mechanical gauge. I would put a new sender on it. It will also be a good idea to service the oil pressure relief valve to make sure it's not stuck partway open and is doing its job, as described in these two threads: thread 1, thread 2.

Also, what type of engine oil are you using?