The FIA have confirmed Mercedes are acting within the rules to use an oil burn limit of 1.2l/100km for the rest of the season.
A rule was introduces saying that any power unit that had not been used prior to Monza could have a maximum oil burn limit of only 0.9 litres, as supposed to the current 1.2l. As a consequence, Mercedes used their fourth power unit the race before Monza, at Spa. This appears to contradict a rule suggesting that engines from works teams must run to the same specifications as non-works teams.
However, it appears the the oil burn limit overrules the other rule, so while customer teams will be obliged to run any new power units with an oil burn rate of 0.9l/100km, Mercedes can carry on running their engines with the current oil burn specification.