Tracy is dead on here. I have a friend who worked at the local Mazda
dealership, and was the head mechanic between the mid 70's and mid 90s. That
was his reasoning for not using it, as well. In a modified engine without
cats, it should be no problem. I run synthetic in the RE-5 and all my other
vehicles, except the 74 step van which I refer to as the SUV (Seriously Ugly
Van).  Mike C.