YM 1700 Oil Pressure Problem

[Missouri Dave]

I just changed the oil (Rotella 15w-40 for Diesels) and filter (yanmar from Hoye). When I first started the tractor at about 1000 rpms, the oil pressure was reading 30-35 lbs. I let the tractor warm up for a few minutes and then went out to mow, when I increased the rpm to 2150, the oil pressure dropped 10-15 lbs. In the past the pressure at 2150 rpms would start out at 45-50 lbs and then decline to 25-30 lbs as the tractor heated up. Any ideas?

Thanks,
Dave