This gives me and idea (for my own use, at least). Having acquired a gas-driven power washer (basically, for cleaning the patio) since last my own oil tank was off the bike, I believe I am going to try an internal tank wash with washer detergent this winter. I think it will do a good job in the bottom where stuff seems to accumulate. For scale on the side walls, MikeN's "handful of washers" approach is probably still necessary. For the gas-tank treatment that I used, the manufacturer instructed using a handful or drywall screws before the treatment. I think this was because they are really hard and have a lot of edges.
Richard L.