In addition to all of the above maaybe the relief valve on the oil pump may be sticking. Also try a qt. of any kind of transmission fluid to the motor oil and drive it for and hour or two. Transmisson fluid has lots of cleaners and such which will remove lots of that sludge and such. You may need to repeat this process a time or two or three. I had a late modle 94 motor with much of the same trouble and noisy lifters. This did the engine lots of good. It quited down the lifter and cleaned to inside of the motor. It did not hurt the motor and I put at least five hundread miles per treatment. Today the motor runs nice and all si quite.
