The announcement comes just weeks after the European Union opened a legal route in its phase-out of carbon dioxide-emitting cars from 2035 which would allow those models with combustion engines running on e-fuels to remain on sale.
The exemption, championed by Germany, was hailed by some as preserving technological freedom but viewed by others, flagging e-fuel's low availability and high cost, as a dangerous loophole for potentially polluting combustion engines.
Stellantis, owner of brands including Fiat, Peugeot, Opel and Jeep, said in a statement it was testing everything from tailpipe emissions to engine power and oil dilution in Euro 6-standard vehicles made from 2014 and into 2029.
Solutions being tested could apply to up to 28 million Stellantis vehicles, with a potential reduction of CO2 emissions in Europe of 400 million tons from 2025 to 2050, the automaker said on Thursday.
"The broad adoption of eFuels would offer customers with existing internal combustion engine vehicles an easy and affordable option to decarbonize their vehicles."
Stellantis reaffirmed it was committed to all its new car sales in Europe being battery-electric by 2030.
CEO Carlos Tavares said last month that the e-fuels vote did not change the automaker's electrification strategy but showed that not everybody agreed with a "dogmatic approach."
"The industry will have to demonstrate that this is carbon-neutral," he said.
The European Commission has until the autumn to come up with a proposal for how to ensure combustion engine vehicles produced from 2035 onwards only run on e-fuels, a process in which it is expected to involve industry representatives.
Chevron and Exxon Mobil said on Wednesday they were road testing renewable gasoline blends but that making them affordable would require government support.