i'm looking to flush out my gf's ATF in her 99 civic ex and was wondering if it's ok to switch to systheic instead of regular? i was thinking of replacing it with mobil 1 sys ones....

 

You may want to call a Honda dealer for sure. They will be able to tell you for sure if you can use a synthetic or not.

 

Yes, you can put synthetic in. You may not see much difference, but it will last longer and could give you slightly better gas mileage. I'm not sure I would spend the extra $$$, though, since you only really take advantage of synthetic's great resistance to breakdown if you are towing heavy loads or doing other things that signifigantly inrease the fluid temperature. The flush would definitely help, though (although you will likely still not notice that).

 

just use Honda ATF fluid....