You said you tried synthetic...for how long? Did you do anything besides drain and fill with synthetic? For an older engine, you should first use a engine flush to get the old crap out. The filter is important as well. That's why I included filter costs.

I don't know what would convince you. I pointed you to the Amsoil website. If you didn't read it, you're asking questions based on your experience with another oil, not designed for extended drains. Then you question my numbers, which were fairly conservative.

Synthetic oils are made for both diesel and gasoline engines. I still don't quite see how synthetic oils are "thinner" than conventional oils. I'm emailing their tech-support to find out what the story on that is.