Filling it with 10w40 makes it run sluggish. I filled my wife's Aerostar 3.0 V6 with 10w40 (actually brewed it up to that with thickeners) and it really feels sluggish like there is a drag on it. On the other hand, when I switched my F150 300 cubic inch (4.9L) inline six to 10w40 I didn't notice any difference. In fact, I added a bottle of VSOT recently so am now up to about 16 cSt and still don't feel any drag. It is obvious to me that the 300 cubic inches has plenty of power to spare and so the extra drag of the thicker oil is negligible, whereas the drag is a major hinderence to the pathetic little old 183 cubic inch Aerostar.