Synthetic, unless you get that 5,000 high milage stuff, does not allow you to safely lengthen your oil change intervals. The additives still break down like the dino oil. Its the same as conventional oil, your engine is just more protected during these 3,000 miles. :biggthumpdakolorado said:
And I'll add to the "free hp" Remove that rubber weatherstrip at the back of the engine bay in front of the cowl. This will allow air to evacuate the engine bay better, thus giving you a cooler engine bay. The rule is one hp for every 20* cooler under the hood.
Also, maybe its in a different place on the older trucks, but that rubber around the radiator on my '05 is to aide in cooling efficientcy for the radiator by directing more air through it. I removed some around the passenger side so my "Cold air" intake saw cooler air, but I'd recommend you leave the rest of it in place.