Would I gain any benefit whatsoever if I switch to Mobil 1 for my 2005 Hyundai Elantra with 21000 miles that already runs butterly smooth on Castrol GTX 5W-30?? I plan on owning this car til 165,000 miles. Im a very conservative driver in the suburbs of central nj..not quite city and not highawy, sort of inbetween. I faithly do 4,000 miles OCI. At 5000 miles, I added Motor Silk engine treatment to oil and the car runs and sounds so smooth on acceleration all the time.