My recommendation of oil is based upon my decades of experience with broadband dummy loads from 60 watts to 2.5 kilowatts. The dummy loads that I have worked with for medium wave and below and from 5 kilowatts down have all been convection air cooled. Broadband dummy loads that I have used for higher powers (up to 25 kilowatts) have been forced air cooled.
I prefer to stick with what I have experience with. As for water, YMMV. Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA http://plymouthcolony.net/starcityeng/index.html
