I was skeptical. I'm all into DIY gifts and the like, but I was concerned about my expensive front loading high efficiency washing machine (which lately has had a very strange moldy smell!). I decided to step off the cliff and try it out. I am hooked. I find myself looking forward to washing a load of clothes. They come out smelling so clean (and, incidentally, so does the stinking washing machine). The best part of it all is that the incredients are variations of bicarbonates and (in my version) a simple, low-sudsing castille soap. All natural. All good. Just fine for high efficiency washing machines.
Here is the recipe that my housesitter shared. You can also Google it for different variations.
2 cups 20 Mule Team Borax (which I could only find at K-Mart)
2 cups Arm and Hammer WASHING Soda (notice I wrote "washing" soda not baking soda)
1 bar of grated soap (bar should be 5-7 ounces...I used Dr. Bronner's bars soap because it smells like peppermint and when it is grated it looks and smells like white chocolate peppermint bark. I don't recommend tasting it unless you are feeling guilty about your use of the F-word and feel you need to wash your mouth out with soap.)
Grate the soap in a food processor or by hand (you want very tiny pieces). Mix it all together very well (I suppose this can also be done in small batches in the food processor).