And an 18 year old truck has terrible mileage...

likely because the sensors in the exhaust/intake are flat wore out. And the fuel injectors don't spray a nice spray like they did when new.

Because it seems that every single auto manufacturer seems to feel the need to use a different spark plug for just about every single engine they make. Then there are probably 3-4 brands that make the specific spark plug you're looking for (maybe more). It's like oil filters. Couldn't all of the manufacturers have just settled on using 10 different oil filter sizes...….