Absolutely right. I meant a small amount of fuel should evaporate. My 'experience' was that, while prepping the Mini for winter storage, with a nearly full fuel tank, the carb float valve stuck open. Little known fact: the top of the fuel tank is higher than the carb. It had all winter (about 4 months) to drain the tank down to the level of the carb bridge. The fuel trickled slowly down into the cylinders, past the rings and into the crankcase. Luckily I checked the oil level before starting the engine - it was at least a litre or two high, and smelled terrible. Now I NEVER store the Mini with a full tank.

If the fuel pump fails it does not evaporate it stays in the gearbox until it reaches the crank seal then it seeps by that and drips onto the ground, ask me how i know lol.
The one good thing that come out of this is the oil changes produce nice clean oil.

Rosebud:   YUP!!!
(checking to see if you read all the way through!)

I assume you meant “if fuel got into the crankcase/transmission.” Correct? The question at hand seems to be: is Britleyland’s alarmingly high oil level reading due to a coolant leak or a fuel leak into the motor oil, or simply a miss-reading of the dipstick, e.g. parking on an incline or taking a hot engine reading when normally he takes a reading on a cold engine.

I was wondering if a fuel/oil mixture produces the creamy mayonnaise that a water/oil mixture produces. You’ve answered my question. The answer is no.  I should have known that. Forgive me, here comes a chemistry lesson… The white mayonnaise stuff is technically called an emulsion; it’s what you get when you mix a polar solvent (e.g. water) with a nonpolar solvent (e.g. oil). You wouldn’t expect to produce an emulsion by mixing two non-polar solvents (e.g. oil and gasoline). 

On the other hand, you suggest that fuel in the oil would simply evaporate and not register on the dipstick. That makes sense too. So, why did Britleyland’s oil level go up? Interesting!

Rosebud. Follow Dan's post. I read one way and answered backwards. Sorry for any confusion.

Rosebud: not clear where you mean re oil and fuel.
If you mean "does fuel in the oil produce a milky colour?" the answer is "no". Fuel will just dilute engine oil... a lot of fuel makes it smell terrible, but still just diluted. (How I know is a long story...) Both are petroleum based (ignoring synthetic oils).

If you mean "What is the off-white creamy stuff on the underside of the oil filler cap?" that looks like mayonnaise, then the answer is an oil-water emulsion condensed on the cap. This is due to cold, humid weather where condensation occurs in the crankcase/engine cavities. When you start the engine, it get mixed with bit of oil and collects on the cap which is a cool part compared to the rest of the oil system. You may also see a bit on the dipstick up near the seal. It looks like mayonnaise because it IS like mayonnaise - a whipped mixture of oil and water.

If fuel got into the cooling system, crankcase/transmission the heat would probably evaporate it and it would eventually be released by the rad cap.

Take some advice from my idol....but its all up to you
https://youtu.be/YL9QjN7AcW8

Does oil and fuel produce the milky color associated with oil and water (coolant)?

As a way to check malsal's suggestion of a mechanical pump failure, remove the valve cover cap. Look for that milky color on the cap and in the valve cover.

Ditto—right away. But if you do have commingling of coolant and oil I would expect that you would notice a distinct difference in the color and possibly viscosity of the oil, especially if as you say the oil was way above the full line. Anything else you can think of that would cause the dipstick to read high? Parked on an incline, perhaps? Or maybe you're used to checking the oil when the engine is cold and this time it was hot? I've noticed a considerable level difference between a cold reading and a hot reading.

If it was coolant it would more than likely be milky coloured by now. It could be fuel pull the dipstick and smell it you will know right away. If you have a mechanical pump sometimes the diaphram goes bad and it with plenty of gas in the tank as it sits higher than the fuel pump it can siphon into the crankcase/gearbox. If this is the case do not drive it change the oil and pump immediately. also if it gets high enough it will leak past the crankshaft seal and could get on your clutch.

I would get after an oil change right away. If your dip stick level is noticeably higher, would most likely be coolant, it would be the first item out of the drain hole, or a cream looking oil. Probably not from the t stat housing but rather a head gasket issue. Good luck 

I've had a small coolant leak around the thermostat housing but have not done anything about it due to the weather. Today I checked it and it was a little low so I topped it off and checked the oil and found the oil on the dipstick was way above the full line. I did drive the Mini a few weeks ago when the weather was nice and put it away and after checking all fluids they were fine. Any chance some of the coolant could have gotten into the oil? As soon as it warms again I will change the oil and replace the housing gasket but it still bothers me if any of the coolant could have mixed with the oil.