In my previous place I started using a contractor portable kerosene, HHO vaporising bowl type heater. That was a while ago, but as I remember I was OK until I went past 25% WMO without preheating, could have been a little less. I tried preheating but I was spending a lot more time working on the heater then on my projects so I upgraded to a used Reznor waste heater with preheat. That system was far superior and allowed me to work on my project versus trying to heat the shop! LOL Even in that system I found that when burning a lot of synthetic which has a lot higher flash point I still needed to blend a little HHO to keep the carbon buildup down. In burning a liquid fuel you should consider that there is an atomzing factor and a flash point factor. Heavier lubricants will have a higher flash point, and are also harde to atomize therefore preheat is a tool, or blending with a fuel thats is easier to light can be thought of as an alternative. Even if you light it the carbon buildup may be excessive and create a lot of maintenance. Maybe others can chime in and help you and more with this.
I have been running a 40% wmo /60% hho in my beckett burner this winter. You Will definatly need a nozzle heater.Beckett sells it for about 32 dollars,Also be sure to start with a new nozzle and filter. Let the oil settle for a day or so before adding it to the tank as this allows the sludge to settle.If you plan on burning straight waste oil you will need a special nozzle and compressed air as it wont atomize well. I plan on going 100% waste oil for next winter also. If time permits ill get working on it soon. Let me know if you need any more info. You will find this group quite helpful but most are burning veg oil.
The pumps don’t seem to have a problem with a wmo mix, in fact it probably lubricates the pump quite well. I have been running wmo and hho in a boiler and at 50% its still necessary to preheat the mixture to about 350°F due to the higher viscosity and higher flash point of the motor oil. In order to avoid preheating you probably wouldn’t be able to go higher then about 20% wmo.