“Excess” oil appears during flight

 There are no means for other fluids to enter the oil system. The most likely cause is the oil system was overfilled and came out of the oil tank vent line. This is normal when the system is overfilled and would suggest revisiting the oil level check procedure - https://www.rotax-owner.com/en/videos-topmenu/expanded-video-instructions/36-exp-si-27-1997

What is the mounting location of your oil tank? At higher positioning of the oil tank, oil might leak through clearances at bearings back into the crankcase and when you do a level check, it may appear to be low, so you add more oil and overfill the system. See installation manual section 79-00-00 pages 14 to 17

https://www.rotax-owner.com/pdf/IM_912_iSeries_ED2_R1.pdf

You want to go back to basics with the oil system design/procedure before we take you down a much more complicated path! It's always best to start with the simple stuff first!

Thanks for the suggestions but I’ve not added any oil since the last oil change about 30 hours ago. Surely if I’d overfilled it that would have become apparent ages ago

Just a thought,

There have been reports of this issue and people did an oil analysis and found fuel with diesel fuel in it. 

To add to what Roger said, there have been other reported incidents of fuel leaking into the engine while the aircraft is parked.  There was a recent post about that happening with a high wing aircraft where the owner routinely left the fuel valve open while the plane was hangared.  On a carbureted engine all it takes is leaky carb float valve.  Not sure if the same thing could happen with a leaky injector but it seems possible.  This would explain the oil appearing “thinner”.  You would likely smell the fuel in the oil.  

Not exactly sure what the problem is but I have experienced two items that may or may not relate to your experience.

Problem #1, I don't always get consistent oil readings, even when I burp the engine. There have been times when I add oil to fill up the tank (after being burped) but when I check the fuel a few days later the oil level is higher and I have to remove some to keep from overfilling. Based on my experience, I sometimes see different levels of oil after I burp the engine. One possibility is that you are inadvertently overfilling the oil due to the inconsistent burping?

Problem #2, after I fired up my engine for the first time, it ran very rough and I shut it down. It was clear from my ECU diagnostics that 3 of my 8 injectors weren't working properly. I removed the entire fuel rail and all 8 injectors and found a significant amount of white crystalline substance in the rail and injectors. After some research, I discovered that it wasn't the remnants of a drug smuggling operation but rather it was likely the dried remains of water saturated ethanol based gas. Where did it come from? Likely the factory uses ethanol based gas during their testing. However, as I was cleaning all the injectors I noticed most of them didn't open or close properly and likely resulted in inadvertent leaking of the injectors. (as an aside, the rotax dealer wanted me to replace all the injectors as well as pay a mechanic to clean the fuel system. They warned me not to touch the fuel system. Estimate was a few thousand dollars! I ignored their advice since I have trust issues and I have experience with a rotax certified mechanic who screwed up one of my parts and cost me about 6 months of downtime. I cleaned it all on my own in a few hours with a fuel injection cleaner jig I made for a few dollars. The engine has worked flawlessly since that time).

I wonder if it may be good to do some experimentation to eliminate problem #1 (perhaps get your oil analyzed). If that isn't the issue, I wonder if problem #2 could be the culprit if you confirm you do have fuel in the oil? 

-Bryan