The technical differences in the oil weights you describe are minimal.
I can't tell you what you can and can't do but I'm assuming that temperatures where you live range into the 90o plus range often. I live in southern Nevada and have used 20-40, 10-40, and 10-30 in my Ford Taurus without any noticeable difference. I put the heavier grades in for spring and summer, then by fall use the thinner. I garage my car so it never gets started in very cold temp's but I have 115,000 miles on it and I've not done any damage to the engine.