Morris Minor Owners Club

Cold engine? Likely.

Worn engine with a lot of blow-by? Likely.

Water leak? Unlikely

I put a fancy aluminium rocker cover on my Ford engine (back in the ‘70s) and it sludged up something awful. I discarded it (probably like the previous owner of it!) and the problem went away.

I’ve seen far worse.

Running too cold.
Check the thermostat for correct function - it should be at least 82 degrees C before opening.
Drain the existing engine oil and fill the sump with a flushing oil and run till the engine is hot and then drain and fill with straight good quality engine oil.
Change the oil filter.
You could run the engine on diesel engine oil as it has a higher detergent content which moves any sludge particles to the filter so keeping the engine innards clean.
A compression check would be helpful.

Do you do a lot of short runs? It looks like the condensation that forms when a hot engine cools is not being boiled off. Give the rocker box a good clean and change the oil. A few long runs should help remove moisture in the oil.

I’d suggest ensuring that the engine breathing system is not also blocked with muck.

Stephen

"I’d suggest ensuring that the engine breathing system is not also blocked with muck."

Good idea. my breathe had become blocked and engine pressure pushed oil out of the rear crank seal.