Mercedes people don't often work on their own cars. That's the problem.

Ok, so here are the only two things that come to mind. First, make sure you have an oil filter with an anti-drainback valve. Second, going to a thinner oil helps cold flow, but it also means that you have a thinner oil that will drain down into the oil pan faster.

I'm guessing your problem is probably just lifter wear, or dirt/accumulation inside the lifter body (probably between the plunger and lifter body or in the check valve seat. Could be a lash adjustment problem, but I'm leaning toward wear or contamination.

The fact that only one or two of them is acting up says that you should probably do a tear down and clean/inspect all of them.
You might want to check out the preload too, which is one of the really common things that makes a valvetrain noisy.

I'm assuming the engine has a pretty decent amount of mileage on it?