My guess is that Ferrari's second oil tank was so they could use two formulations of oil at the same time, possibly in variable ratios to satisfy different engine modes.
Technically this wasn't in breach of the regulations which did (and still do) refer to multiple oil tanks (though there is some distinction made around the area of a 'main' oil tank) and I'm pretty sure that at the start of the year there was specific mention of 'no more than two approved oils per Event'. That has changed with article 20.3 of the technical regs now stating that 'only one oil may be used in a given engine during an Event'.
Xwang wrote: ↑29 Aug 2017, 22:41
How will FIA measure the oil consumption?
Have they already decided or will they invent some other fantastic rule.
From the Technical Regulations...
"7.8 Main oil tank level measurement:
The measurement of the oil level in the main oil tank must be supplied to FIA at all times. The main oil tank is the oil tank directly connected to the engine oil feed at the inlet of the oil pressure pump."
and from the Sporting Regulations...
"34.9 The mass of oil contained in each oil tank, with the exception of the main oil tank, must be declared to the FIA one hour before the start of the race."
So, despite the wonderfully draconian sounding but in fact meaningless 'at all times', the FIA will receive information of a certain (possibly variable?) quality from the teams at some point and then do with it what they will.
(Sorry if you're after the technicalities of how such measurements are achieved - I haven't a clue but I'm guessing they don't use a dip-stick.)
IMO, the FIA should inform the teams that the oil consumption figures will be made public from Monza onwards, to coincide with the introduction of the new limits (for all teams/engines on at least a per session basis).