If you have a stuck lifter, a sticky ring, or chronic problems with black death (oil sludge) there is a very good way to sort it out, which works in most cases, and has been used on my 944 twice in its long life (220,000 miles on the original engine without a rebuild), once when I first got her when the lifters were chattering through it not having enough oil changes, and once about 30,000 miles ago when I got back to the country and she had been standing for 2 years since my previous journey in her! So it will not hurt the engine at all..
What you do is mix cheap and nasty 20-50 oil from a super market, in a ratio of 50-50 with diesel.
Drain your engine oil, give the engine a fresh filter, fill her up with this recipe, start her up... while the engine warms up, follow a cycle of letting it idle for 5 mins, then lifting the engine speed to 2000 rpm for 5 mins.. do not drive the car during this.. keep an eye on the oil pressure, and time how long it takes for the radiator fan to come on.... after the radiator fan has turned off, keep cycling the revs as mentioned for just as long as it took from starting the engine until the fan came on and shut off (you can do it longer than this if you want..
Once you have done this, drain the oil mix (leave it draining for an hour or so), change the oil filter and give fresh good quality oil, something like 15w50 mobil, and take her for a carefull drive... in 9 out of 10 times, this will cure the problem unless you have a completely knackered lifter.
Its a bit like a mega engine flush on steroids to keep in the arsonal as a "if all else fails" idea.
It scares the hell out of most people... but you need to remember, diesel is oil, just a very thin oil, so it thins down the 20-50 oil so it can creap into clogged parts or defeat air locks within deflated lifters. Also diesel contains deturgents, which help the process of moving sludge.. the only reason we use 20-50 is that it is cheap, sufficient and gives about the correct thickness when thinned with the diesel.