I would take apart the complete rocker arm assembly and look at each rocker. Check it for wear on the inside and then check the tip of the rocker for waer. Both of these will cause problems. Is the shaft that the rockers run on wore or cracked. Are the oiling grooves turned the correct way.
This is one reason geting new shaft and rockers is always a good ideal. Make sure that you are not losing
oil around the four stands where they bolt to the heads. Look for to much
oil loss around all of the parts. I assume that this is a completely adjustable rocker arm system. We still like to get the valve stem heights similiar as well. I would assume that you have no valve spring coil bind. Just all possible questions and I do not know what all you know about engines.
If you have fairly high spring pressure the lifter will have a harder time staying pumped up with thin
oil so heavy oil may by the answer just depends on the short coming on the design.
Good luck, Keith Craft