2015 has the flawed, direct injection only system. The intake valves are prone to coking and carbonizing. If bad enough, you get a 'check engine' light. It is covered under the 8yr/80k emission warranty. The only true fix is head removal. Foaming the engine, sends the gunk and carbon chunks into the turbos and catalytic converters(nightmare time). Invest in an extended warranty. Some shops do a 'pan drop' and filter change. That leaves about 7 or 8 quarts of dirty fluid behind. Ford dealers offer both flush and/or drain. The system is back flushed with 95% new fluid. My expedition has a drain plug on the converter. The shop replaced around 13qts. The previous owner had it flushed, the pan was spotless and clean, the old fluid was pinkish-brown, at 85K. The new fluid still cherry red after 5yrs. The red however, is just a dye. You can buy it for $5.