Your oil pump gears may be worn to the point that they allow the prime to slowly dissipate over time. Your oil pressure will be lower than desired also. You can replace the gears with new ones. They are not hard to find and it is easy to do. Also, check your oil pump cover for wear and scoring. If the scoring is not too deep you can sand it smooth. Otherwise, repair kits are available that provide a spacer plate that goes between the cover and the housing on the front cover, which will increase your oil pressure. Hope this helps.