Greetings folks, new to the forums and this is my first post.

I'm stumped.

I have a 1996 Civic DX sedan (5 speed manual) that was getting 29 mpg (based on 28 full tanks) in the city. I changed the oil and filter from conventional to Amsoil synthetic oil. I instantly lost 7 mpg (based on 7 full tanks) and my check engine light came on. I had the car tuned up (plugs, rotor, fuel filter etc...) as instructed by the garage I took it to. The tune up produced no results. Any ideas? I though synthetic oil was supposed to improve mileage not drastically decrease it.

Thanks in advance guys.

RA

 

There is another problem, what is the check engine light for? Ive used full synthetic in my 92 civic forever with no problems. The oil wont affect the gas mileage that much.

 

Update:

The check engine light came on again today. I am going to stop at Autozone on the way home and have them diag it for me. I always believed that the Amsoil / bad mileage thing was a coincidence, this hopefully will prove it out.

 

Do you work for Mobile 1 or something?

There is nothing wrong with that oil, something else happened to your motor.

 

Nope. Nothing better than Amsoil. Just wondering if 10 year old cars + Amsoil was bad. As I mentioned I am all but convinced it was a coincidence.

 

You wouldn't be able to notice a difference in mileage between different oils like that. I'd suspect with the drop of mileage and the MIL light coming on it could be a possible o2 sensor. That'd set the mil off and your mileage.

 

i switched from conventional to full synthetic Supertech Walmart 5w-30 a couple weeks ago.

i still get good gas mileage.

mix of city and highway got me 260 miles on 7 gallons = 37 mpg

 

Bad Oxygen sensor it is. Being replaced today. Thanks for the ideas/opinions and help.