I checked the vent hose running off the cover on the carbs into the crankcase - no blockage. There was some moisture in the hose - small amount. Don't think this is an issue.
If you're sure the entire breather hose is clear of blockage the next thing you can do is run the engine with the breather hose detached. First top up the engine with oil, up to the upper mark on the dipstick. If you run the engine with the breather hose detached, you should see a lot of smoke coming out of the valve cover exit. Check if there is still oil coming out of the dipstick opening. If yes or if there is no smoke coming out of the valve cover, take of the valve cover and check the inside (the oil/air seperator tube) for blockage. You can do that with pressurized air, fluid from a degreaser, a spraycan of WD-40 or some thin oil (engine oil or ATF). If there is no blockage and engine oil is still coming out of the dipstick, do a compression test or a compression leak test.
If oil no longer comes out of the dipstick, try running the engine with the breather hose attached. If the oil comes out again, check the hose still further for blockage.
Warning 1: carefully monitor the amount of oil you are losing, running the engine without oil is not good, so be ready to turn the engine off.
Warning 2: blow-by gas coming out of the valve cover is not healthy, so do this in a well-ventilated garage or outside.
Do you live in a cold climate?