Iv`e just ordered 10 bottles of millers CVL, and now iv`e been told it`s not a good idea to use this if you`ve still got the cat on your car, is this correct or is it safe to use it on my STI4, 1998? I have got a HKS hyper back section.

 

You've been told right. CVL has a VSR (valve seat resession) inhibitor in it and will knacker cats. This stuff was primarily designed for older cars requiring leaded 4 star. It increases RON more than standard octane boost which is I guess why people use it/advertise it.

 

Just ask your supplier to swap it for millers octane+, which is ok for cats