What oil filter and fuel filter I use. And also what oil I need.

Not only does this YouTube video show the location of the oil filter on a 2001 Chevrolet Blazer, but it also claims that an entire oil change can easily be done by an average individual with a few common tools.

The oil filter is place underneath a flap on an easily-serviceable part of the car.

Watch the video for more specifics:
Doin an oil change on 2001 chevy blazer part 1

Blockage in a oil gallery --thats for sure but where ,its a strip engine out and rebuild job to find it ,what about the oil pressure relief valve on the oil pump

look under the front of your blazer, just below the bumper, and you will see a little access door in the plastic guard. Open this door, and you will find your oil filter.

change the mass air flow meter

along the frame rail just look along the frame it's cylindrical:)

check for the fuel pump shut off ,in the rear pannel inside the truck!!!!!!

replace the fuel pump electrical wire harness.

Check the oil if you haven't already. If that makes no difference, change the tranny oil and filter. It can make a world of difference. Another thing it could be is your fuel filter. The engine is starving for gas, and can't shift properly.

Usually when it comes to bad gas mileage, an air filter change, fresh oil and filter, fuel filter and making sure you have no vacuum leaks....

Also, believe it or not, improper tire pressure will affect your gas mileage.

New spark plugs and wires are a "nicety" but if there is not damage to them, then they will be fine.

I would recommend you call Schram auto and truck parts at 1-800-292-1032 and see if they can assist you since they sell only GM parts