I have read several comments about the climate control system being problematic on '86/'87, 300 SDL models, among others. It appears the fix is to convert to "134." What does that mean?

A thorough explanation including ballpark estimate would be greatly appreciated. I live in Phoenix, AZ...

BTW, I am in the market for a well maintained acquisition per recommendations from this forum. I also plan to run biodiesel at the optimum ratio. Any suggestions?

--Will