But is anecdotal evidence good enough for this pseudo-scientist? Well, actually, yes, and if I had a HE washer, I'd feel comfortable using it based on those recommendations. But I had some time to kill this evening, so I decided to dig deeper.
According to this Cleaning Institute pamphlet, the reason there's specific soap for HE washers is low water volume. That's partially how HE washers are so efficient – they don't have lots of water to heat up. But regular laundry soaps are sudsy. Lots o' suds + not much water = sudsy clothes that don't get very clean. The suds can also clog up your HE washer. Boo. The pamphlet also mentions special formulas that hold dirt in suspension in the water so it doesn't re-dirty your clothing, but that's only mentioned briefly, and the point that's hit over and over is the suds. So we'll test for suds.
I took two bowls, filled them with an equal amount of water, and then added a teaspoon of laundry soap to each. On the left, we have the bowl for homemade soap. On the right, we have the bowl for the Seventh Generation soap, which has the little HE symbol on it, telling me that it's good for HE washers. (And non-HE washers, too.)
I agitated each bowl simultaneously for 60 second. The results?
The DIY soap had almost no suds in it. (The few that were there popped before I could get a picture in.) The HE soap had a low level of suds, too. I wish I had some non-HE laundry soap hanging around for an even better comparison, but alas, I don't. (And I can't bring myself to buy even more detergent, given my current abundance.)
Anyway, the moral of the story is: DIY laundry soap is very low-suds, and should be safe for your HE washer.
Also, these mini-experiments are a blast.