I posted this on another thread but I thought I would just start another post. I am changing oil on my dodge cummins every 5k miles with conventional rotella and mobil 1 filters. I drive about 15k to 20k a yr. If I change to rotella synthetic 5w40 can I safely extend my drains to 10k? If not, what would be realstic? I do not plan on adding a bypass filter or anything like that. Just wondering if this is safe. I cannot justify going to synthetic unless I can extend the changes out. Conventional oil has worked quite well for my brother. He has 2 dodge cummins with 500k and 250k. They have seen nothing but a steady diet of conventional oil. The only thing I am trying to do here is extend the oil drains out.