3 Point Lifter- KAMA 254C

Things were going fine last time I used it but this time when starting up, then going up a slope, oil began spurting out the lifter fill plug which is vented. I hadn't added any oil since long ago so not overfull.Can it be a blocked filter or something in the cylinder that is on the side of the lifter box? As I had to get the grass mowing job done and it was on flat ground I continued on and the lifter worked OK after the initial squirting out of plenty of the oil.

Also on checking the fill level for that lifter reservoir, do you screw the dipstick/filler plug right in before unscrewing to measure the oil level or should the plug be just rested on the top to measure? It makes a big difference as the plug is an inch or so long.

Any help much appreciated