They eliminate propeller torque as a reason for the aircraft to have a critical engine.

When a plane has two engines, the one that will most affect the plane's handling if it quits is called the critical engine. If a twin loses one engine in flight, the plane will turn toward that engine. But in a critical-engine situation, the plane will turn more sharply toward one side. On a clockwise-prop plane the port engine is critical; the starboard engine is critical on a counterclockwise-prop plane.

This doesn't necessarily mean the plane doesn't have a critical engine. A plane might have one hydraulic pump. If the engine running it dies, you're going to have to strong-arm the plane back to the ground.

No. If your plane has counterrotating propellers, and the props turn toward the fuselage, the plane doesn't have a critical engine. If the plane has counterrotating props that turn away from the fuselage, or both props turn the same way, the plane has one. There's another "critical engine" situation that comes into play even if you've got counterrotating props: if a critical function (cabin pressurization comes to mind) is handled by only one engine, the engine that drives it is critical...then again, it's not that expensive to put a cabin pressure pump on both engines, so that's not real common. One that will make you go hmm...the Diamond DA-42, which drives its props through a gearbox, has a critical engine. They couldn't have put a selectable reverse gear in it? Simply put, the Critical Engine is defined as the engine whose failure would most adversely affect the performance or handling qualities of an aircraft. If, as pointed out above, neither engine would have a greater impact on handling, then neither would be considered the critical engine.

Diesel contains more impurities compared to gasolene.Therefore fuel filter is more critical in a diesel engine

Aircraft engine has some critical problem.

Eventually one of two things will happen. 1. The oil will sludge up and clog up the engine, then all the critical components won't receive oil and the engine will seize. 2. The engine will use all the oil, then all the critical components won't receive oil and the engine will seize. In the end, bad things will happen regardless