I also feel that it is not unusual for a slight delay of a few seconds before seeing oil pressure on the pressure gauge. Both my 2N and 8N have a couple of seconds delay.
But 15 seconds does seem like a little too long. Are you sure that it takes that long? Sitting on the seat watching that gauge can seem like an eternity.
You may want to consider using the lower viscosity oil during winter time, as TOH suggests. I also use Rotella 15W40 in my tractors, but they don't see much winter use. I would use 10W30 if they did, and put the heavier weight oil in during their spring servicing.