Hi Martin, I've written quite a bit on this previously, but I'm not sure if/where Bert files all the posts, so maybe I can add a little more to K Bike's recommendations (as we are mostly on the same page it seems).
The K engine holds 3.75 litres of oil in a finned crankcase sump. It has a large oil pump when compared to similar and larger engines. The K engine (as you know) is also water cooled and the cooling system is well over designed. Put simply, the whole engine is extremely robust and over-engineerend in typical German fashion, that's why they last for ever if well maintained.
The biggest "killer" of oil in our K engines is condensate, which will be generated when the engine is only used for short periods (less than 15 minutes), particularly in colder climates. There are no high temperatures anywhere in a properly maintained K engine, the maximum oil temperature will be around 85 degrees C, and this will only ever be encountered in hot weather, in city conditions. Open road, 2-up fully laden you will see around 65 degrees C, trust me I have measured this with an infra-red thermo gun.
Excessive temperatures shorten the oil's life. Boxer engines have this issue because of their cylinder head temperatures. So, to summarise, Synthetics are required in order to cope with high temps, other than that you are wasting money really.
I use 15W40 Diesel oil (Caltex Delo Silver here in Aust), and change it and the filter at 5000 K's. My engine has 167,000 K's and uses no oil between services. Hope this helps, Dennis