Mitsubishi Repair: Montero engine knock, vacuum leaks, fuel injection system

Question
I have a 2001 Montero with 3.0 engine and 140,000 miles. There is an audible engine knock that corresponds to engine speed. Oil pressure is fair (30# at speed, 5# at idle0 and no oil light has been on ever to my knowledge.  I have had the engine oil changed to 10w40 and have treated the engine with Rislone engine treatment.  I still have the knock.  I have been driving this truck for several thousand miles and the knock gets no better or worse.  What should I try next?

Thanks for you help

Answer
Terry: What kind of fuel are you using in this car? Something with corn mixed in it? As much as we want to save a bit of money at the pumps, you need to be putting in a bit high grade ot fuel. No corn mixed fuel, as that is really hard on some car's engines.  But when you fill your car up, don't use the cheapest fuel, try something more costly and remember it will take a while for this to work. As you have to get all the old fuel moved through the system. Ok, I am pulling this right out of my repair manual for things to try, after incorrect grade of fuel, ignition timing incorrect, fuel injection system faulty, inproper or damaged stark plugs or wires, worn or damage distributor compoments, EGR valve not functiong and vacuum leaks. So try the fuel idea first and see where that gets you, if that does not stop the knocking then do or check these others. God bless: Linda