Helicopters have flight manuals The only reliable answer is to comply with it
Just checked the TCDS re deisel for the French Astazou and Artouste engines and the following are approved with associated temperature limits, so they were operating in accordance with the flight manual. All legal.
Automotive Diesel Oil DCEA/21 C
- VVF 800 DF2 TS.10.003 F54 Not to be used at OAT below -5° C
- VVF 800 DF1 Not to be used at OAT below - 15°C
- VVF 800 DFA F56 Not to be used at OAT below - 15°C
Gasoil O 7120 STM MIL-F-16884 DEF 2402 (47/0 DIESO) F75 Not to be used at OAT below -5°C
Gasoil 20 7120STM DEF 2402 (47/20 DIESO) F76 Not to be used at OAT below 0°C
Illuminating Oil DCEA/11C VV-K211 DEF 2403 F58 Not to be used at OAT below - 15°C