As happens every year, Blue Opal’s oil needed to be changed. The club doesn’t have the vacuum pump any more (a replacement is coming), but I was able to borrow a small brass pump (archive) that did the trick. Very simple to use, just had to get the depth into the sump correct; I ended up offering the dipstick up next to the suction tube, and applying a small bit of electric tape. My only worry was the tape decided that the oils on the outside of the tube were sufficient to get the tape to slide off and into the sump.
The biggest issue I have with the oil change is actually the position of the oil filter on the 3GM30F – on the side of the engine, just ahead of the engine mount, and underneath the alternator. It always leaks oil when removed, because it’s full of oil even after the sump has been drained. However, this can be sorted with some oil/fuel absorbing cloth – old oil still spills down the engine casing, but with a cloth in place, it’s trivial to clean up. Just over a litre drained out, so I’ve definitely been burning oil – a bit over 2 went in before the dipstick registered full, but the engine is meant to take 2.7, so I presume some of the old oil was still deep in the sump. Managed to source 20 litres of Castrol oil too, so I should be good for at least 8 years from that one container!
Catching all the oil 
Oil down the casing
According to the Yanmar parts manual, the relevant part is 124450-35100, which has been superseded by 119305-35151, which has been superseded by 119305-35170. I just used a Blue Print 72101; it’s what the club uses on the 3GM30s in the launches, and they’re a fiver at the local car parts store.