PTU = Power Transfer (or Takeoff) Unit. It's the part that controls the AWD distribution. When the AWD Explorer's and Tauruses came out in 2011/2012, Ford said this was a lifetime fluid unit, meaning you never needed to worry about drain and re-fill. However, over the years, they've had all sorts of trouble with the bearing in there starting to wander, and it makes contact with the housing, resulting in failure. The best way to help prevent this is by changing the fluid regularly. Some people say at 40K - 50K miles, but I'm going to be more cautious with my 2016 Taurus SHO and do it every 20K miles.
At least with these 1st Gen 3.5L Ecoboost engines, they are a very dirty, messy animal. Fuel dilution and oil deposits are quite common. Right now, I"m going to use Valvoline SynPower at 4,000 mile intervals max. SynPower is known for low calcium, and also meets all the needed spec's for turbo engines. I am also considering just using Motorcraft syn blend at 3,000 mile intervals. I drive mostly short trips, and fuel dilution is something to consider.
Regardless of the oil you choose, it is crucial you stick with 5w30. I have personally talked with Ford engineers and been told it is critical to stick with the recommended viscosity in these engines due to the turbo requirements. I believe when the 3.5L first came out, they did recommend 5w-20, then changed it to 5w-30 a few years later. Keep in mind a variety of the 3.5L has been in the F-150's for a lot longer than the Taurus SHO, so a lot of the issues have been discovered quite some time ago.
From everything I've learned about the Ecoboost engines, it's Best Practice to use a quality motor oil which meets spec, is of the recommended viscosity (5w-30 in this case) and do frequent oil changes.
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