I'm running a chinese 495 diesel on an ST-20 direct drive. I run 75% WMO and 25% #2 fuel oil blend. I use fleet oil that I pick up locally with my home grown tank trailer with an old oil truck pump, a 25hp Onan engine, and a 200gal horizontal oil tank. Here's how my setup works. I blend the raw WMO and the #2 oil in the storage tank. I bring the oil blend unfiltered into my run tank passing it only through a water seperator. Then it goes into my heated prefilter system.
I use a plate type stainless steel heat exchanger heated by engine coolant. before the fuel enters the heat exchanger, it passes through a 12V three way solenoid valve. This allows me to leave the system full of #2 FO at shutdown so it'll start without ethering it to death. Next it goes on to the first stage prefilter. This prefilter is nothing more than a felt type filter commonly used on a residential oil burner, it's commonly known as a "general filter". The replacement element can be had for under $2.00 and needs to be changed monthly under continuous use. This, of course, depends on the cleanliness of the waste oil.
After leaving the general filter, it goes into a 10 micron Garber spin on oil filter. Again, this is something that's designed to use on residential oil burners. I put the Garber optional restriction guage on top of the Garber filter housing so it shows me the restriction in the first stage and the second stage at the same time. Any time the restriction gets out of the green scale, just change the first stage filter and it drops right back down. With about 1000 hours on it, I've not yet had to replace the Garber filter, but I like the assurance of having a 10 micron filter in the prefilter.
From there, it goes through the stock 2 stage fuel filters that came with the genset. Once it reaches the injector lines, I put a Carlin nozzle line heater close to the injector on each injector line to heat the fuel again. I attached these little ceramic heaters with spring clips that I bent in the vice and covered the joint where the heater meets the line with red silicone to avoid heat transfer into the air.
I start the engine on #2 fuel oil, let it run until the coolant temp is 70C, then flick the switch to the WMO blend. It works great. The only time I ever had any problems is when I tried to run straight WMO in it. I noticed that the exhause was starting to show some light smoke and running in #2 didn't clear it up. I removed and cleaned the injectors and went back to the 75/25 mix. 500 hours later it's still going strong and makes no smoke whatsoever. Come time to shut it down, I run it for 20 or 30 minutes on #2 fuel oil to clean the WMO mix out of the system and make for easy starting.
This won't work in a two pipe system unless you use a holding tank after the prefilters to put the return fuel through or it's just going to plug up the first stage prefilter very quickly. My generator set has a one pipe system, so this makes it a lot easier. If you add a tank, you'll need to vent the air / vapor out of it continuously with a float vent or similar device, but it should work just fine. If you need to vent, make sure to run the vent line back to your holding tank, so if the float valve gets stuck you don't make a mess.
raw fuel blended in the holding tank -> Water seperator ->Three way solenoid valve -> plate type heat exchanger -> General oil filter -> Garber oil filter -> both stock oil filters -> injector line heaters -> then into the engine.