BMW Repair: 540 lifters worn?, synthetic motor oil, hydraulic lifters

Question
Hello Evan, I have a 1999 540 with 150 km always Bmw serviced by first owner, I am now the second. I have noisey lifters at idle when the engine is up to normal operating temperature. The noise tends to go away when the engine rev's higher than idle. They are quite when the engine is cold. I have tried an oil flush to clean them, it did not solve the problem. It sounds as if only 1 or 2 lifters are noisey on each bank, about 1/3 the way long the head from the front. Some one said to me, it is a big job to replace them, and best leave it alone.  The noise really bugs me, and would love to have it put right. Is it a big job, can you just replace the worn ones, or best to do them all. Have you had this problem before with the 540? It's a shame, I reckon the bmw oil service interval is too long and has caused them to become worn. I have thought of running diesel motor oil for a while, hoping to clean them, and maybe fluke fixture, if they are dirty. What do you think, can not harm to try first?  
Thank you for your advice and opinion.

Answer
Richard, it is indeed a big job to replace Hydraulic lifters. Yes you can replace the lifters that you feel are 'soft' but I would replace all 24 so you don't have to go back in and do it all over if you miss one. Is oil pressure within spec. I would substitute a quart or two of ATF and see if that helps at all. I wouldn't do this for an extended period of time. I recommend maybe a few days at max. I have seen this on M62 V8 engines such as yours. Make sure you are running a 5w-30 full synthetic motor oil and have changed the filter and checked pressure first.
-Evan