Although I've only encountered lifter noise on the v engine, the symptoms do sound similar to sticking lifters caused by too few ,or too long between, oil changes, thanks to vauxhall trying to impress fleet managers with longer service intervals.
Usual advice is to try a few quick oil changes, with wynns, to clear out the goop. So it might be worth persevering with another oil change or two at least, ESP if the oil darkens quickly, as it's taking away deposits in the engine. And hopefully flushing out your lifters.
If suspecting oil pressure, it's best to check the oil pressure output with a suitable gauge by removing the oil pressure sensor, fitting the pressure gauge and running the engine to take a reading. Haynes has the correct readings, and then see of the pressure is low when cold and hot.
How long since the oil change, and how many?

There is a v engine on here with low oil pressure, and it does rattle. But only when hot, presumably when the oil is thinner. So the opposite of yours.
So maybe persevere with the oil changes and wynns. It's known to work. But ultimately it could just be a duff lifter. While the noise is worrying, if it is a lifter, going by Marks comments on here it's not great deal to worry about, I believe.
It's a futting tick (as opposed to a "tocking" tick) suspect exhaust manifold. They then seal up when hot and go quiet in early stages of failure.
Hth
In answer to your last post. No.
