Generally speaking - don't know why a Merc would be different. Wrist pin = tick, at least until it gets very close to going and should get louder under load or when backing off. A lifter will just hammer, scare the hell out of you, and may seem like it's missing.
If you oil is old, I'd first pour in and additive to see if the lifter frees up, and then switch to syn.

__________________
89 300E
79 240D
72 Westy
63 Bug sunroof
85 Jeep CJ7
86 Chevy 6.2l diesel PU

"The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane."
Marcus Aurelius