Most synthetics are fully compatible, so you can go back and forth or even blend. As for the hum, since it's audible during a coasting turn, it could very well be gear noise. When you're decelerating the wheels often turn the driveline, which means the back-side of the gear faces get engaged. Since they're not worn in like the the other, normally driven side it'll make noise like a fresh gear set.

This is just an idea based on experience, so I can't say for sure without hearing yours in person. But it's something to think about.