I do it all the time. I like the product, and like the price better. I get Delvac 1 ESP 5w-40 for cheaper than any synthetic on the store shelves. Obviously, one should be cognizant of one's choices, though.
What's your application? If you're using M1 0w-40 already, I'm not sure that you're in need of an HDEO. It could, however, be a feasible option.