Here's a wee tip to help you get the most out of your items. Upgrading the highest items may seem like a good idea but it's not always the case. For instance Out of these two it may seem like upgrading the Ace Fighter would best however they're both going to increase your squads attack and defense by just 2. The training plane in this instance is the best to upgrade! Why? They're more common making them take less time to upgrade and as they're more common I have acquired 5 of them. This means that in 14 hours less time I can increase my squads attack and defense by 10 instead of 2. So check your inventory you'll often find you have multiple items that you're using. Hope this helps!