I'm currently running Castrol RX 15w-40 in my 02 VW Jetta 2.0 and it runs good. But I hear that the modern 2.0 engine has low tension pistion rings and tighter engine clearance. So VW would reccommend a thinner oil for fuel economy. I'm running 15W-40 to reduce my oil consumption. So my question is that by running a 15W-40 in this engine, will there be any long term effects on the rings or the catalyst. Also how much change is there in the 2.0 engines of the A4 VWs compared to the previous generations? I know my wording is not accurate since I don't know too much about engines. Thanks for any replies.