None of the non "American customary" units are in use, anymore, to my knowledge, other than troy oz's in the gold industry, definition of foot in land surveying, and pints in a british pub. But if anyone wants to add something that isn't ancient history, feel free to show us where Americans are using slugs and stones and confusing tower oz and imperial gallons. There are probably "old units" used in farming still, in many countries for estimating weight of stuff in the field. But that is not anyone's concern other than the farmer.
Oil barrel vs. fluid barrel vs. barrel barrel barrel?
Even in here news often quote raw oil price in usd per barrel, even if hardly anyone has idea what barrel they are talking about and how much it is.
"Maximum accuracy when converting bbl to cubic metres
When used to denote a volume, 1 bbl is exactly equivalent to 42 US gallons and is easily converted to any other unit of volume. A volume of 1 bbl is exactly equivalent to a volume of 158.987294928 litres.
In the oil industry, following the definition of the American Petroleum Institute, a standard barrel of oil is often taken to mean the amount of oil that at a standard pressure (14.696 psi) and temperature (60 °F) would occupy a volume of exactly 1 bbl. This standard barrel of oil will occupy a different volume at different pressures and temperatures. A standard barrel in this context is thus not simply a measure of volume, but of volume under specific conditions. The task of converting this standard barrel of oil to a standard cubic metre of oil is complicated by the fact that the standard cubic metre is defined by the American Petroleum Institute to mean the amount of oil that at 101.325 kPa and 15 °C occupies 1 cubic metre. The fact that the conditions are not exactly the same means that an exact conversion is impossible unless the exact expansion coefficient of the crude is known, and this will vary from one crude oil to another.
For a light oil with an API gravity of 35, warming the oil from 15.00 °C to 60.00 °F (which is 15.56 °C) might increase its volume by about 0.047%. Conversely, a heavy oil with an API gravity of 20 might only increase in volume by 0.039%. If physically measuring the density at a new temperature is not possible, then tables of empirical data can be used to accurately predict the change in density. In turn, this allows maximum accuracy when converting between standard bbl and standard m3.
International commodity exchanges will often set an arbitrary conversion factor for benchmark crude oils for financial accounting purposes. For instance the conversion factor set by the New York Mercantile Exchange (NYMEX) for Western Canadian Select (WCS) crude oil traded at Hardisty, Alberta, Canada is 6.29287 U.S. barrels per cubic metre.,[21] despite the fact that crude oil cannot be measured to that degree of accuracy. Regulatory authorities in producing countries set standards for measurement accuracy of produced hydrocarbons, where such measurements affect taxes or royalties to the government. In the United Kingdom, for instance, the measurement accuracy required is ±0.25%.[22]
Qualifiers
A barrel can technically be used to specify any volume. Since the actual nature of the fluids being measured varies along the stream, sometimes qualifiers are used to clarify what is being specified. In the oil field, it is often important to differentiate between rates of production of fluids, which may be a mix of oil and water, and rates of production of the oil itself. If a well is producing 10 mbd of fluids with a 20% water cut, then the well would also be said to be producing 8,000 barrels of oil a day (bod).
In other circumstances, it can be important to include gas in production and consumption figures. Normally, gas amount is measured in standard cubic feet or cubic metres for volume (as well as in kg or Btu, which don't depend on pressure or temperature). But when necessary, such volume is converted to a volume of oil of equivalent enthalpy of combustion. Production and consumption using this analogue is stated in barrels of oil equivalent per day (boed).
In the case of water-injection wells, in the United States it is common to refer to the injectivity rate in barrels of water per day (bwd). In Canada, it is measured in cubic metres per day (m3/d). In general, water injection rates will be stated in the same units as oil production rates, since the usual objective is to replace the volume of oil produced with a similar volume of water to maintain reservoir pressure."
So oil barrel is 42 gallons and water barrel is 41.5 gallons except when water is injected to oil field its same as oil gallon =42 gallons