Last time I had squealing, I used some WD40 on the back of the tensioner and never had another problem with it.

 

Gates Green Stripe XL (or Napa Gold which is the same thing) is the only way to go. They're a little more money than the cheapies, but they'll last longer and you won't have worry about how the belt is holding up.

 

I tried a fancy GY belt, but it would squeak when the numbering patch would cross over the waterpump pulley(since it rides on the back side of belt). I ended up with an Autozone version and it is nice an quiet.

One way to keep tabs on the belt stretch and how tight the tensioner is, take belt off and relax the tensioner. Take a black sharpie and put a line across the two halves. Now install belt and you can watch as the two lines get closer together as the belt stretches. For a more permanent mark, use a punch or chisel.

 

I'm still getting random sqeaking mostly before it warms up, sometimes it takes 10-15 miles to stop. A new tensioner pulley may be in order.

 

My 92 F150 5.8 was quiet until I swapped a 3G. That thing howled like crazy. I tried WD-40 to no avail. I replaced my belt with a nice Gatorback one and the noise decreased significantly but it still squeaked some. I wound up replacing the tensioner and the tensioner pulley with new ones from Motorcraft and it has been silent ever since.

 

These belt tensioners come with those marks right from the factory...