T140e Oil Pressure

This is my first post and I'm hoping that someone can help.
I have just finished a complete rebuild of a 1979 T140E. I have fitted an oil pressure gauge to the oil pressure switch outlet. The bike is running well after a few minor niggles, and I've just completed the first 50 miles since rebuild. The only issue that I have is the oil pressure when the engine is warm. When the bike is started from cold the pressure gauge indicates a fluctuating 45 to 60 psi at tickover. As soon as the revs are up the pressure steadies at 60psi. After 12 to 15 minutes of riding the pressure starts to drop, which is expected as the engine and oil are heating up. After 20 minutes the pressure is still dropping down to an indicated 25 to 30 psi. It seems to stay steady at that. I'm using 20W50 oil and have a paper filter in the bottom of the reservoir. It has a new oil pump. There are no apparent leaks and the bike feels good with the engine very responsive.
Am I being paranoid or have I got a real problem? The manual says 65 to 85psi at normal running, but as the relief valve operates at 60psi I'm not sure if its correct. Any ideas?

 

I agree with you. I never used to have the gauge and rode blissfully ignorant of oil pressure. The pressure seems to stabilise at 25-30psi so I think thats where it should be. I know what you mean about the oil seal, I learnt years ago what happens if you fit it the wrong way round. I had a lot of oil coming out of the breather!

 

This is the bike