I have a 1992 mazda mx3 with a 1.6 sohc 4 cylinder. I decided to run synthetic oil in it. How many miles can i put on it before i have to change the oil again? Will the filter last just as long (a normal Fram filter)?

There is quite a lot of discussion on this subject back a few years if you search back through the pages at the bottom of the screen. It might change your mind about using full synthetic oil in an older engine, or even at all!

If you go back to page 66 down the bottom of the main page, you will find a post titled "Synthetic oil or regular", it might be of some interest to your situation!