I've used MMO for years and am very happy with the product. I use it in the gas, in an inverse oiler, and for the last 500 or so miles of an OCI interval. The amount you put in the gas is 4 oz per 10 gallons of gas.
As far as shelf life, as long as it is red in color the product is fine according to the company. If you visit the Turtle Wax website it will answer a lot of the questions about the product.
As far as cleaning injectors you are better off with a product like Techron, and once the injectors are clean MMO will keep them clean, lubricated, and act as a top oil. For a really sludged engine ARX seems to win hands down.
Frank D