Lifter Not Pumping Up

If it comes down to pulling the intake manifold to get to the lifter, I'm just going to replace the set... possibly the entire valve train.

What I would like to know is, should I warm the car up, letting run for several minutes, to see if the lifter unsticks? Or does that risk damage? I've been too afraid of the clatter to allow it to run like that for more than the couple minutes I have, so it's gotten no where near running temperature.

Today I observed that more lifters than just the #3 intake are squishy; it seems about 6 of them are, to varying degrees, though none as severe as #3 intake. It's entirely the forward lifters too; the ones that haven't been bathed in oil for the last couple of weeks while it's been sitting.

Admittedly, I cheaped out on my oil filter... it's a fram. I'm afraid this might be part of the problem: bad oil. However the oil and filter were changed less than 3k miles ago, about 15 months.

My other fear is that my oil pressure is poor. The pump is old, of unknown origin, and I only have an idiot-light to tell me what the pressure is.