Richard,
I have a Bukh DV 20 on my Seal 28 (Saphena Minor)and change the oil every season. I hope the following is helpful:
You need to get the engine thoroughly warmed up to make the oil thin enough to pump it out. This will take up to half an hour or so of running the engine so, if you intend to do it while the boat is out of the water, you will have to jury-rig a supply of water to the water pump inlet before you start, and make sure the outlet is clear and doesn't throw water all over the boat next to you! It is best to change the oil at the end of the season just before the boat comes out of the water to avoid this problem (and also to allow you to run the engine in gear, which is better for it). Old engine oil can be corrosive, and this is another reason why it is best to change it at the end of the season, rather than leave it in the engine over the winter. Given that we are already at the start of February, however, you might consider leaving the oil change until you put it back in the water, assuming that willl be around the end of March. It really depends on how easily you can rig a cooling water supply while it's on the hard.
If your boat is anything like mine, you will not be able to drain the oil using the sump plug - you can't get at it. You will have to pump the oil out of the engine through the dipstick hole. To do this, you will need a vacuum pump with a probe, which you can buy from a chandlers - they don't cost too much. For the pump to work, the oil must be thin, which means it must be hot. It takes a while to pump out all the oil, so don't be in a hurry.
While you're at it, you should change the oil in the gearbox as well, which is another good reason to have the boat in the water, so you have it in gear and warm the oil in the gearbox. On the DV20, the gear box uses the same oil as the engine.
After consulting Bukh and Shell, I use Rimula R3X 15w-40, which has the correct viscosity for these old engines. You can get it from Lubricant Supplies on the internet. When refilling the engine and the gear box, be careful to use the exact quantities stated in the engine manual.
Once the engine is empty, change the oil filter and tighten the new one properly as Mike says, with the right tool. I can say from experience that cleaning up after a leak from a loose oil filter is a very messy and unpleasant business!
While you are changing the oil, take the opportunity to change the sacrificial anode at the back of the engine - it is important that this is done every year. Also change/clean the fuel filters and replace the water pump impellor (always keep at least 2 spare impellors on board.
Hope this helps.
Dennis Sewell
Saphena Minor