96 Civic + Synthetic Oil = Lousy Mileage?

[rwsanderson]

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

[96Cex]

The two are unrelated. The oil did not cause low mileage or the check engine light. Just a coincidence.