Rough idle in 520i E34/M50

Hi,
I have several problems with my 520i and will appreciate any help to evaluate them before looking at it or going to the stealer.
The car is an 520i, bought new by me in 1994, engine M50, 154,000 Km, european
specification, bought in Spain and I had used only 98 octane petrol due to its lower sulphur content, the car still has the factory exhaust in good shape.
First problem is that at idle I can feel some engine vibration, I have also a 320i with the same engine that it's a lot smoother, no perceptible vibration when inside the car.
As it's easy to see from the mileage, I use the car only ocassionally but regularly. Lately, after sitting in the garage over a week, it runs very rough after starting, like if a cylinder was not firing, after warming goes better but the small vibration stays.
The front of the engine is noisy, with a muffled rattling sound in the area of the vanos. It has been always somewhat noisy but this worsened after the last oil change in which the dealer used the Castrol syntetic 0W-?? oil now specified for current cars, I complained and they even showed me a notice from BMW telling that this oil is fine for older cars. I'm still not convinced.
Can you give me your suggestions please?
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I would look for a vacuum leak/air leak on the intake. I might also change back to a 10W-40 or similar oil. 0W was never recommended for those engines.
FloydR
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These engines sometimes suffer from sticking valves. A compression test should show this. Other possibility is a tracking coil. Assuming all the usual things like hoses and EGR valve have been checked.
--
*Acupuncture is a jab well done*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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