Ok guys, I got hold of Matthias and at first, here is the BRISK spark plug type that he recommends for LT. It is : BOR14LGS
Secondly about the mineral vs. synthetic oil: As I suspected there is really no one truth in this subject. I asked Matthias the following question: "The engine oil. If I understood right you said that BMW recommends mineral oil for the K-engines. Can you explain more about this?"
He usually answers me in German but this time he used English: "The engine oil is a mineral oil with a viscosity of 20/50.
Mineral oils are for example always used in new engines, because the first kilometers with a new engine critical is always.
Here one can use only the best engine oil. As one takes this simply also after the entry time and one always is on the safe technical side."
So...how do you want to interpret that? I guess there ere already enough oil related threads on this forum, let's not make this an additional one. Everyone is using his/her best oil and that's it.
And clarification about the oil filters. During my 110.000 km ( 68350 miles) I have used probably 4-5 Champion C 301 filters and perhaps 4 HiFlo filters in addition to 1-2 OEM filters. I change oil and filter every 10.000 km (6000 miles).. As I already stated with this frequency it is probably very unlikely that an oil filter (even without a bypass valve) would cause an engine failure. And whether non OEM filters have this valve or not - I personally do not know - I just repeated what Matthias explained to me. My decision is to use OEM filters from now on.
Then about my mysterious oil consumption:
I started this season Apr 4th with 100631 km (62529 mi) in my odometer. I had new semisynthetic Mobil Super 2000 10W40 motor oil and new HiFlo oil filter plus brand new K&N air filter. I had filled the oil level to about 75% of the oil sight glass as I always did.
I took off for my Central Europe trip May 7th with 103150 km (64094 mi) in odometer. Before leaving I checked the oil and noticed that it was at half of the sight glass and I added about 1dl (3,4 oz) which I never had to do before. I did not pay much attention to this at that time, I just figured that maybe I just left the oil a little low 2500 km ago when I filled it. And besides, I was perhaps going to ride a bit faster on the German and Dutch autobahns, so let's make sure I have oil.
At 105630 km (65635 mi) i.e. 2480 km (1541 mi) later I was at the servce with Matthias and my oil sight glass was dry. This distance included crossing Sweden and Denmark as well as riding around in Dutch small roads so riding hard on the German autobahns was actually just some 100 - 150 km max. (And besides...my throttle only opened 80% at that time so the "hard" was not so hard after all...
Matthias filled the oil sight glass to 95% full. And he changed back the OEM air filter. I took the attached picture yesterday and my odometer read 109480 (68027 mi). So after 3850 km (2392 mi) my oil level had dropped to the level you can see (is it 80 % or so?)
This distance includes at least as hard driving as my leg before the filter change. (Now I get full power and I have been enjoying it..
Therefore again, I just repeat what this Matthias (whom I consider and expert guy) explained to me. I have seen with my own eyes how my oil sight glass has dried out from 60-65% full to zero (or God knows how much below, because the oil level was no longer visible) in a relatively short distance (1541 miles).
After changing the OEM air filter my oil consumption has come back to normal.
Yes, I have now new spark plugs, new spark plug cables, new fuel quick connectors, new oil filter, new oil sight glass AND new rubber intake manifold sleeves. As I mentioned two of them had already cracks on them but at least on the eye the cracks had not gone through the rubber yet. I also had the K&N filter changed back to OEM. Out of those new parts my bet goes for the K&N air filter as being the cause for the abnormal oil consumption. But also, as Toolman says, a leaking rubber sleeve could cause similar symptoms. (Actually I would think the consequenses of this should be much more dramatic, judging from the sample piston and cylinder Matthias showed to me.)
At least my conclusion is to stick to OEM air filter from now on. I'm really wondering what was I thinking when I went away from it in the first place. They cost about 30% more than OEM over here but perhaps I was figuring that I would save money by not needing to change it.
But...each to his own. Thank you all for your comments and interest. I've chosen my way...