Hello all

What is the best air filter that I can buy and put in my 2008 Focus 1.8 TDCi that will increase MPG and not harm the engine long term. Any suggestions?

The simple answer is none will make any noticeable difference. You'll get more MPG by doing 69mph rather than 70mph on the motorway than you will by changing your air filter to a different type. Not to mention the cost. A high end panel filter will cost way more than what you could potentially save in MPG gains.

Going back many years I used a K&N or Pipercross as they outlasted the standard filters at the time, never saw any other benefits. Nowadays OE filters are usually very efficient and last well, so I stick with those.

The paper ones from Halfords have worked just fine for me, but if you want something nicer, a K&N would do it. They're washable so you can just keep using the same one (which hopefully justifies the price!) If it's a mk2, pull the airbox lid off if you want more noise and more accessible air, though I can't see it would improve anything as the intake sound muffler (idk what it's called, the snorkel thing) was designed to let the engine run efficiently. Probably not such a good idea now we're getting into winter though, you don't want water in or near your filter and intake.