Oil boiler issue.. Poor Domestic Heat?

I have a Magic Heat boiler for forced hot water heat and domestic water. I have had a few year of performance issues and my oil company that I have a contract with is only turning up the Hi/Low (So i just figured out). My issue is not with the house heat as that is great though with them turning up the heat, my bleeder valves seem to be leaking. Anyhow, I was looking over the boiler and they have it set to 220 and that seems REALLY high and the oil use has gone up considerably over the past years. After looking deeper, I see that the mixing valve is leaking water (white milky) and I think that my issue is that the valve and the boiler need attention. I found stuff online to clean the coil and I also found some replacement scald valves but don't know if I should do the work. I have no issues with stuff under the sink but am unsure what I need to know if any before attempting this myself. Advice would be awesome..