I have 4 qts of each. Is it ok to mix or should I just stick with one weight?
As an AMSOIL Dealer you can mix the oils. Keep in mind you are mixing a Signature Series oil (more robust additives package) and an XL oil. If you are going to do this run it no more than the XL oil recommendation, 10,000 miles or 6 months. My recommendation is order five more quarts of 5W-50 and use the 10W-40 for something else like your lawn mower.
So are you trying to make a 7.5W-45? lol