The oil pressure will vary by how hard the engine is working. At 600 RPM, the oil pressure should be about 14-15 PSI. As the RPMs climb the engine is working harder so the oil pressure will climb with it.
The engine can hold about 5.2 qts. (4.9 L) of oil.
The plugs on either end of the camshaft occasionally come loose, causing the oil pressure to drop. If the engine is being worked up to 2,000+ RPM and the pressure remains unchanged at around 15 PSI, loose plugs might be to blame. Replacement plugs can be purchased through Kubota and can be attached by welding or use of a sealant.
The rated performance, or maximum workload the trailer can handle while still remaining viable is 2,800 RPM. At this performance, the pressure should be between 40 and 50 PSI.