I picked it up by accident when it was sale at Meijer. I've been mostly using Synthetic PenPlat/Ultra 5W30/10W30 on all my vehicles (2 cars, a Honda Odyssey, and a RAM 1500 Truck). All have high mileage, the lowest is the Honda with 130K. The truck and one of the cars have some seal leaks and I've been planning to switch to HM oil on the next OCs. Anyway, I only have this 4 qt jug and if I decide to use it on the truck, I need another 1 qt of something (I have plenty of 5W30 Pen and G-Oil Syn) can I add 1 qt of 5W30 Syn to the 15W40 HM oil or should I just use it on one of the cars (though both recommend 5W30)?