>  I haven't been using synthetic either based on some bad experience we've 
  had at work with synthetic oil in vacuum pumps. Here's the story: We 
  switched to synthetic because it was less affected by some of the chemicals
  that get run through the pump. It worked better and gave easier cold starts
  too. Later, all of a sudden we start having big lubricant problems, like oil
  getting thick, black and stinky in 48 hours (we usually change every 4000
  hours). We call the oil vendor in, they confirm the problem and blame it on
  Mobil, who makes the feedstock. Mobil won't take any responsibility saying
  "the feedstock *only* has to be good enough for automotive use!" More 
  specifically for those of you that are oil techies, the problem was related
  to the oxidation resistance of the oil which is created by "hydro-treating"
  (whatever that is). Anyway, to get to my point, I'm not using it because
  it IS synthetic and some drunk, stupid, or stoned human is in control of 
  how good it comes out. In 25 years of changing oil (Castrol mostly), the
  dinosaurs have never, ever let me down. Those big lizards really knew how
  to de-compose.....

  BTW, when Mobil 1 first was introduced, their big claim was that it didn't 
  have to be changed as frequently. I have no quarrel with that, but in an
  engine without an oil filter, like an upright fan air-cooled, leaving the
  oil in more than 2-3 Kmiles makes me uncomfortable.

  Synthetically Yours,

  Dave Kautz