Is this stuff as good as other HDEOs? Anyone use it? This summer, I am putting a few cars on 15w-40 and am considering the RX. My mechanic uses *it in every Euro car that does not require synth. It seems like a good choice for customers when you don't know when they will be back for a change and it meets specs too. *I don't know if he uses actual RX, but it is a 15w-40 Castrol HDEO in-a-drum and he said some thing about it being formula 3 something something.