I took my time setting the preload and making sure valvetrain geometry was correct. Didn't give any attention to the lifter oil feed height compared to the lifter bore feed hole. Guess that could be the issue.

So, how much preload did you set? Maybe try a bit more or less. I can't see any manufacturer setting oil hole at the wrong height. I doubt even a big change in base circle would move hole far enough. Plus, I would think if oil hole location was off, it wouldn't change with rpm. Its not like the lifter moves farther at 2000 than 1200. And these are retrofit lifters so I doubt they are for a gen 6 block, which I bet has same oil gallery height as 4-5 blocks anyway

This may open a can of worms as this has most definitely happened.

preload is .030. Motor has around 100hrs on it from previous owner. Other than valve guide/seal wear on the original heads everything looked good. I did pull a lifter and checked rollers. All looked good. I'm out of time to wrench on this thing with my job picking up so a complete tear down won't be possible for a while. Thinking about teeing off the oil gauge tap with a needle valve and run a copper line to valve cover and make a spray bar by drilling small holes in it. Total rig, but I need to get out on the water to keep my sanity. Lol

I just compared gen 6 roller to mk4 flat lifter. The cut-out around the waist is about an 8th inch lower on the flat mk4 lifter. I am sure people run retro lifters in 6's when the dog bones would restrict lift, but I have never heard of a retro lifter made specifically for a gen 6. You bought engine used. He might have bought lifters used. Loosen lifters up to no preload. No better? Tighten down to the max they can be set at. No better? Replace the lifters. Pull 1 and put next to flat tappet lifter to compare oil cut-away location just in case, but these lifters just ain't pumping oil. Another thought. You are looking at 1 lifter. Pull valve covers and run engine. Messy but... it might be a couple of gummed up lifters.

i can see all the lifters between the fill cap and breather holes. They all have the same volume of oil coming out of them. I have to pull the exhaust off to remove the valve covers, also there is a b&m 420 blower bolted on top so it's a chore to disassemble for access to the lifters. I have around 200hrs in this boat doing a new transom, rebuilding outdrive and rigging motor in. I'm out of energy and time. May have to wait till work slows down and disassemble it.