Outer Limits 4x4 Board

hi all i have a small problem with my lwb 83 sierra it seems that my sump seems to be getting pressurised and spitting oil out thru the dipstick hole i was just wondering any ideas on what it good be as well how to fix it

blocked breather valve

Rings could be shot , this is called blowby and one of the indicators are air being pushed out dip stick dragging oil with it , also pull oil filler cap off , if it puffs out lots of fumes you might need a rebuild , Cheers Paul.