I have been running Amsoil signature series in my fleet of 35 cars for 11 years. The difference is clear to me. For use the gains aren't in horsepower. The engine protection is excellent. We have vehicles with 400k on original motors, the only reason we get rid of them as the body style has become dated (we're a black car / limousine service)

The 2nd benefit, and this surprises some people, is cost. Let me preface this buy saying we buy it in bulk (30 gallons drums). However, a 7 qt oil change with an Amsoil Filter is about $78. The oil and filters we use are rated for 25k miles. I change them every 15k. If someone were paying labor on their oil changes a 7qt oil change with conventional or synth blend would be in the ballpark of $30 at a cheapo Jiffy lube type place. Even at 5k intervals that's $90... so it actually costs more. On top of that you had to get it done 2 extra times for those 15k miles.

Lastly we saw a decent gain in fuel economy when I took the time to run the numbers. I don't remember the exact numbers now because I did it back when we first considered making the switch over. But with the number of cars we run and the total mileage it was a significant savings.

I'll get off my Amsoil soapbox. But I always sing its praises when I get an opportunity. As the fleet manager and sole mechanic for this facility it has made my life very very nice. Our average sedan does 70k miles per year, so you can do the math on how often I would be changing the oil with shorter interval products.

For the 4.0 the Amsoil filter would be a EA15K51 - That's at 15k service interval filter.

I would pair that with their 5w30 Signature series oil for the best protection and 15k service interval, or the XL series oil and change it at 10-12k instead.

Cheers