It's something of a mystery at the moment, Mick. If you remove the cap from the oil filler tube with the engine running, does it puff much?
Briggs recommends SAE30 oil, not multigrade, but it must conform with API standards such as SG and its successor standards. It makes no difference whether it is synthetic or not. You are currently using 20-40 multigrade, which will be increasing the oil consumption compared with straight SAE30. However this doesn't usually cause oil to come out from sealing surfaces. At the same time, I have to point out that a side valve Briggs engine is not a Honda: I've never personally seen one that was actually oil-tight, though I don't doubt they exist.
Please report back on how much huffing and puffing goes on with the dipstick removed, then perhaps we can talk about whether there is actually anything wrong.
