Dude, I think all that 45mph vibrating you're subjecting yourself to has got you addled!
Our engine can't have "clogged" or sticking lifters - because it doesn't have any lifters! (Thus the Direct Attack moniker) If your valves are tapping/ticking, you just need to have the valve lash adjusted. Are you SURE they checked your valve clearances? Did they pull the valve covers off? If not, you might want to get that checked again. In the good ol' days of rockers with screw adjusters we used to say "a noisy valve is a happy valve" since it's better to be a little loose that a little tight (cost you a smidgen of power but that's better than burned valves) With the VX, however, you wouldn't want to be *too* loose since we have SOB not SUB shims. The Shim is Over the Bucket, not Under it, which makes adjusting the valve clearance easier but the downside is there's the potential that at high revs the shim will get spit out if there's too much clearance. Then you have a disk of really, really hard metal bouncing around in the top end and that's not a good thing. I imagine it would look like a very expensive foosball game, where the ball is the shim and the cam lobes are the little kicker-men. There would have to be a lot of clearance for the shim to make it over the lip of the bucket though so performance would suffer long before you got to the point of destruction. Who knows - it might even be a self regulating thing - i.e., if there's enough valve lash that the shim would pop out at high revs, the engine won't rev that high. Engineers can be smart like that sometimes.
I hope you find that bulletin. I'd like to read it. I think the VX manual stated a valve check/adjustment interval of 60,000 miles but then Isuzu backed off that and said no need to delve under valve covers unless there is a noise or performance issue. Sweet! That means it can wait until timing belt change.
One thing though - you said it was a "hamming-kind-of knocking". Did you mean hammering? Because I hate to tell you if it's a lower pitched hammering/knocking sound, that's NOT valve train noise - and it most definitely won't be helped by that watered-down 0W oil!