Oil light stays on too long on a cold start

My 81-87 225 has an issue with the oil light staying on too long when you first start it, around the same length of time you see after an oil change. This engine is very sound, it's a rebuild from a parts store and has about 15,000 miles on it (no oil usage, no blow by) and I'm using 10W/30W oil and a Wix 51806 filter. After start up the light never comes on and goes right out on a warm start. It appears to me the oil is draining out of the filter or maybe an oil pump problem??? Any ideas??