There is a bored oil gallery past the center of all the lifters that supplies oil to them. It sounds like you've got a clogged pushrod, or a metering valve in the lifter that's not allowing oil to go up the pushrod. Try putting about a pint of kerosene or diesel fuel in the oil, get it up to running temperature, and run at a fast idle for 30 minutes to an hour. Change oil & filter while still hot. If that doesn't clean it out, it will take a new cam & lifters, probably more than you want to spend on a rehab job.
Jerry
