I've dissected a few old mechanical oil pressure gauges,and inside them there is a half circle coil of flattened brass tubing (Bourdon Tube ?) ,that tries to straighten out as the pressure goes up,and a set of gears connected to it and the needle is what gives you the pressure reading..
I guess the brass tube might be liable to leak after exposure to ethanol gas,its only soldered shut on one end and the solder might not hold up against it for long too..so that is probably why they are only rated for oils and not fuels..
Motor oil is flammable,I have scar tissue on my right thumb thanks to some igniting with a loud "WHOOF" when I poured a little into a wood stoves air intake pipe in my garage one night...but its flash point is something like 400+ degrees..solvents and fuels like gas can be lit by a spark at a much lower temperature..I'd rather have a oil leak on my engine than a gasoline leak any day..
I almost lost my '69 GTO one day when the rubber fuel hose on the in line gas filter I put on it ruptured,and the car still ran.with gas spraying all over under the hood--I smelled gasoline very strong while sitting at a red light,I got out and opened the hood,and the fiberglass mat under the hood was soaking wet and dripping with gas!..
I shut it right off and sat there blocking traffic a good 15 minutes,after I stopped hearing drops of gas sizzle when they hit the exhaust manifolds,I tore that insulation mat off and threw it on the side of the road..I bet if I had driven it the 1/2 mile to my house,it would have gone up like a roman candle..
