[SEL] When is it an oil field engine?

[Curt Holland]

Guys,
Have a question for you. What exactly defines an engine as a oil field 
engine?
-It's not the number of cycles.....I've seen a few (very few though) 4 
cycle oil field engines.
-It's not the HP.....I've seen oilfield engines in the 4HP range.
-Some have side shafts and some don't......
-Some have hot tube and some are mag fired.....

Is it the NG fuel use? (Did any OF engines use a fuel other than NG?)
Is it simply that it was marketed for use only as an oil pumper?

Looking forward to your thoughts....
Curt Holland
Gastonia, NC