Mpg ???

[Diamond B Ranch]

I know, it gets asked all the time, but here goes mmy rendition. Our 2000 crewcab 4x4 auto has lost mileage from a previous of around high 17 to 14 MPG. The truck is stock with no mods and the fuel filter has been replaced twice in the last 6 months since we noticed the slip in mileage. My wife drives it as her daily driver and her habits have not changed. We currently have BFG AT's that are about 1 inch taller than stock and the truck has 189k on the odometer. We just got it from the shop. They got no codes and said the sensors all seem to be reading. The mechanic ran the engine self diagnostics several times and occasionally got "#8 cylinder not contributing". He emphasized that this was not a continually thing. AS far as I know the engine has had no work done to it so everything except the water pump is original (minus filters, oil...). Could the #8 cylinder thing be a precursor or indicator of bad injectors or is there another possibilty to explain the loss of mileage. The only other contributing factor I can think of is, that I ran a re-useable air filter for a while and when I cleaned and re-oiled I think I over oiled it. There was excess oil in the bottom of the air box. Could this be causing the sensors downstream of the filter to be sending bad data and not showing as bad?