Mayonnaise type deposits in the oil filler tube can also be caused just by condensation. After my mother died my farther got into the routine of taking his Fiesta out of the garage, driving 3/4 mile round trip to the paper shop, leaving the car on the drive all day then put it back in the garage in the evening, with maybe a couple of short trips out of no more than a couple of miles during the week and a 30 mile round trip to do his shopping once a fortnight.
He checked his oil and fluid levels fairly often and as the Fiesta never used any oil to speak off he never took the filler cap off. After a year or so he asked me to give the car a service for him, on removing the filler cap the rocker cover was full to the top with light grey sludge with the rockers and push rods having made their own little holes in it to move in.

Location: Pembrokeshire.