I'm familiar with the old 617 (early '80s) diesels which have, what
appears to be a hold down clamp to secure each high pressure line to the
injection pump. That hold down clamp not only keeps the high pressure
line in place but also adjusts the injection pulse for that cylinder's
injector. Your more modern engine may be different in this respect but I
mention it here so you understand that tightening such a hold down clamp
may have further implications than first appears.
When you changed these high pressure line "O" rings did you delve into
the IP at all?
The 617 diesels use an engine lubricated IP, there's an oil supply line
to their IPs. Your engine may be so too, take a look.
If your IP is engine lubricated, which it probably is, any defect in a
seal between fuel and lube oil is your engine's problem, IMHO.
I can't offer much more help and suggest you take a look at and pose
your problem to the DIESEL forum or group on www.mbz.org
These diesel owners are knowledgeable and may include a few that own the
same model as you do and have hands on experience, for if your engine
leaked at these "O" rings, then others also had the problem.