It's a transit time ultrasonic flow meter. Entrained air (bubbles) or suspended solids are the enemy of transit time, scattering the energy in the ultrasonic 'beam'. An alternative like a PD meter means the medium is not likely dirty with suspended solids, but might well carry air bubbles. Caveat Emptor. Transmit time likes pure liquid.
Other brands of Ultrasonics require about 10 diameters upstream for flow conditioning. I suspect EFM does too, whereas PD meters need no upstream conditioning.
Transit time accuracy depends on the meter knowing the sonic velocity of the medium at the medium's temperature (sonic velocity is very temperature dependent). Knowing sonic velocity when the medium is water is easy (lots of published data on water), but it is much more of an unknown when the medium is vegetable oil or isobutylpentylhexgonal additive (that's a made-up name). Can the user provide a sonic velocity for the material? No, sonic velocity is not published on the MSD sheet.