The leaders were relatively evenly matched through the first two stages of the Thursday evening loop, but Citroen team leader Loeb pulled clear on Mynnila.
His team-mate Mikko Hirvonen and Ford's Jari-Matti Latvala share second place, the latter frustrated with his form so far.
Mads Ostberg got off to a very strong start in his Adapta Ford and is just 0.2s down on Latvala and Hirvonen in fourth overall.
World Rally Car rookie Thierry Neuville is also impressing in fifth, 1.5s behind Ostberg.
Ford's Petter Solberg started strongly and was second for a spell, but a slower time on SS3 dropped him to sixth.
Returnee Chris Atkinson holds seventh place in the Citroen usually driven by Olympic medallist Nasser Al-Attiyah.