Washing soda...it isn't marketed here in Canada using that term, at least on the East Coast, and I don't remember it in either Quebec or Ontario. I have an idea it is a "US only" term. Here on the Island where the deep well water is mineralized, and surface wells are very soft, you can get a product such as Calgon Water Softener which contains sodium sesquicarbonate and sodium tripolyphoshate. Not the right stuff, I'd say. When you Google it, there are a lot of questions about this from Canadians wanting to make their own detergent or to use in photo film developing etc.
I did some research and it seems that washing soda is sodium carbonate...and even found a way to make it from baking soda (sodium bicarbonate) heating it over a stove, or preferably, outdoors with a propane barbeque in small batches. Not going to do that! A lot of the commercial stuff of various sorts (swimming pool stuff etc) has perfumes or other additives in it, listed or unlisted. Don't want to be putting that in an engine without knowing exactly what is in it. Will be visiting Bulk Barn, Canadian Tire, Sobey's and Atlantic Superstore, and Wal-Mart to see if anyone has this in a reasonably unadulterated format.