Hello and welcome to the forum.

I will also very much agree to stick with oem parts. I bought my t100 about 6 weeks ago and also had some bad gas mileage. I have cleaned the maf with cnc maf cleaner ($4), changed the spark plugs (densos from dealership $2-3 each) as well as all the belts ($100), hi flow water pump ($100), pcv valve ($3 part), ECT sensor ($30 part & found out after removing it was fine) and oxygen sensors (both for $60). The 02 sensors I tried getting away with universals but they threw a code so had to order a denso or ntk and I went with the ntk 24554 because I found some really cheap and that got rid of the code. I have gained roughly 3mpg but have only ran one tank so far but am pleased if thats all I get since that will up it to 18mpg+.

Next up are the spark plug wires which I should have done instead of the ect ($@#%*! autozone) and those will definitely be oem and can be found for around $30-40.

Densos or ngk/ntk for plugs and those 2 brands will work for almost everything on these trucks and always worth the few extra bucks and in some cases they're cheaper. I know you only asked about the plugs I just thought I'd share my experience in gaining more mpg since my truck had the original plugs at 135,000 miles so routine maintenance also wasn't in my previous owners vocabulary and I have become pretty familiar with my engine in the last month.