I used to run 87 as well but have now changed...lets look at the science behind this. The compression ratio of the endeavor is 10:1. Typical cars are 8:1. A higher compression ratio calls for higher octane gas. Reason behind this is because when it say 87 octane, it basically saying that the gas is 87% octane, and 13% heptane. Heptane has a lower compression ratio, meaning it will spontaneously combust (without a spark) at lower compression.
87 octane gas is good for compression ratio's of 8:1. 91+ octane is good for engines with compression ratio's above 8:1. If you operate the Endeavor with 87 octane, you basically are subjecting the Piston, connecting rod, rod bearing (etc)in the engine to premature combustion due to the heptane content of the fuel. So as the cyclinder is in its compression cycle, the heptane will spontaneously combust which prematurely applies force in the opposite direction on the piston which can eventually cause damage to the engine.
Gas mileage doesn't really mean crap. It is about the combustion characteristics of the engine, and the compression ratio should dictate octane. Running lower octane in the Endeavor will lead to lower horsepower, lower performance, and could eventually damage the engine. Running intermittant 87 (if 91-93) is not available should be okay.