The HPDI engines (leastways according to my Yam dealer) do not need the "fancy" stroke oils that Mercs and Ficht do. They use standard TCW3. I agree the oil is a significant cost and the lack of it a benefit for 4 strokes. Personally though I wouldn't want the additional weight penalty - taking the figures kindly supplied by Manos the Yam 4 stroke equivalent of my HPDI is another 70 kgs!
The new 2 strokes are in a different league to old carb engines.
Of course as Julian says we can all go for diesel, providing of course you want half your boat taken up by your engine, can afford £5k a pop for a new Bravo 3 leg every 150 hrs, have beefed up the engine mounts to cope with the weight, don't mind lifting the engine out to do any major servicing and are sanguine about the inevitable imposition of full duty on marine diesel in the next few years
Optimax came from the Orbital engine corp of Australia. Their tech has never made it into cars. Probably down to the same conspiracy that says that boffins have developed an engine that runs on water but the oil majors are supressing it