OM603 hydraulic tappet questions

[Carlos0422]

I have a couple of questions about hydraulic tappets in the 603.970 engine. These tappets, having been used in IC engines for a very long time, seem to be a pretty good idea but I wonder why Mercedes didn't incorporate them sooner if they are indeed superior to solid valve lifters? That's my first question. Secondly, does anyone have experience on maintaining a 603.970 engine with hydraulic tappets? Do these tappets experience any issues that require ongoing maintenance? Is there a mileage interval that one should check them? And, since oil must enter the lifter through a small oil channel, does the oil weight have any effect on the operation and/or lifespan of the lifter? I've never had a car that didn't require manual adjustment of valve clearances (I'm either very old or very poor or both). I know this is probably very basic to most of you but any information you could share would be appreciated.