Recently I have twice come across the problem of oil in the coolant overflow reservoir.  Once in the southern hemisphere and the other in the north. 

Initially there were concerns that the oil/water pump seal had failed leading to contamination of the coolant system.  This was where collective wisdom debated the possible cause (Will, Olaf, Bob & I).  I am now convinced this is not the case (I blame cheap French wine for this).

However when the coolant was drained via the alternate system of disconnecting the rubber pipe attached to the water pump, there was clean coolant on both occasions (Remembering that the oil will form a film on top of the coolant/water mix).  This can confuse the issue because as soon as the engine stops, the oil will separate to the top. 

I would expect that with the volumetric output of the pump (unknown flow rate) that there would be odd specs of oil caught up in the system due to turbulence and being trapped at lower levels, especially within the radiator when the engine stops. (The water and oil pumps being direct drive off the engine crankshaft) 

However when draining the radiator of all its contents I came across a small amount of oil within the radiator.  It actually took me weeks, after removing and flushing the radiator to get rid of this as there are five (?) external connectors, so I had it set it at different angles on the work bench to completely drain everything. 

Why oil in the radiator?  Will put me onto the solution for this because of a conversation he had with the PO of his 75 that I borrowed.  As his 75 had been sitting for a while and was owned by someone with limited knowledge of motorcycles (and none of BMW K series), a third party has offered advice on how to top up the oil.  Because the oil level is easy to see (& check) but the filler cap is not obvious to all. Especially as most second hand Ks come without a Rider's Handbook (and toolkit), there was no obvious way for the uninitiated (and unwashed) to find the oil filler cap. 

As a general rule most buyers of Ks nowadays are a bit older and some have experience of British motorcycles in the days when the oil filler tank was external to the engine.  As such, to some people, the coolant reservoir could be mistaken for an oil tank.  More so as there is a rubber hose running towards the top of the engine.  Meanwhile if the coolant level decreases then oil will eventually be sucked into the coolant system via the radiator cap and into the whole system.  On Will's 75 this has not occurred but on mine the whole system has been contaminated. 

Back to the oil in the radiator, and I have I put this down to the amount of oil added to the coolant tank.  If a little top up was done, then there would be no effect and this could be removed by simply draining the coolant tank.  However as our unwashed and uneducated POs have observed no change in the oil level, they have continued to add oil, expecting a change in the sight glass.  This would have continued for how long until their peanut brains realised nothing was happening.  Judging by the black oil I drained from the engine of my 75S/2 then there was never any oil change done by the PO!  From my assessment of him, there was zero maintenance performed during his ownership. 

In June whilst in Austria, Will through the Flying Brick forum arranged for the legendary Georg to provide assistance regarding another matter.  I was staying at Hotel Post, Galtur, Tirol (exceptional food presentation and complete motorcycle garage) and we discussed the oil in coolant issue.  I was assured that due to the design of the engine, oil cannot enter into the coolant side of the system. 

Two days later in Luzern, Switzerland, Rene and Ronnie drained the coolant system and there was no sign of oil in the coolant system below the radiator cap. 

Fast forward to September and I was speaking with a good friend of mine in the automotive industry.  Topics included Bosch fuel injection systems, fuel pumps, head gaskets, oil and coolant mixing.  As the K series engines are based on a BMW's automotive experience, the K design is taken from that arena.  From this it was explained to me that coolant will always leak into the oil system (99.9% of the time) and it is quite rare for oil to enter the coolant system.  It would probably be through the head gasket and only if there is sufficient pressure to force it through at the top of the engine.  Assuming that the engine design allows for this. 

Sooooo I am now satisfied that the cause of oil in the cooling system is due to oil added in the coolant reservoir.  I have also learnt that a complete oil/water pump overhaul kit will cost you 200 monopoly dollars Euros from a French BMW dealer.

If the drain hole from the oil/water pump is blocked, and the oil seal on the pump fails, then oil will get past the water pump seal into the coolant. I have seen it on two bikes, my own K100RT, and a friends K75RT. The drain hole on mine was blocked with dirt and on the K75 with silicone sealant. It is not a lot of oil, but it ends up in the expansion tank, and is easily seen. If the drain hole is not blocked, then the oil will leak out, and not into the coolant.