So according to AMuS; the engine situation is as follows:
- the engine Haas is using is a specification 3 engine. Ferrari engine 4 has not been homolgated yet. According to sources, Ferrari is unable to find the expected performance increase on the dyno, so has pushed the introduction of the new engine back. Haas's 4th engine is therefore specification 3. If they want the newer engine, they will have to take a 5th engine.
- Mercedes has its version 4 engine (3.1 they call it) since Spa. That means that all customer teams will also get that one. Due to not having enough ready, Force-India and Williams will have to wait. They can either have 1 engine each in the next race or wait until Mercedes is ready to supply them with 2.
- Mercedes and Ferrari spoke to each other because Ferrari felt cheated due to Mercedes bringing its new engine in Spa. Apparently the 0.9l/100 directive is a bit ambiguous because it just speaks of engines and not engine-specification. One could interpret it to mean that Mercedes could use the old directive because their engine is already in the pool. On the other hand, it could be understood to mean that all engines must confirm to the new directive as of Monza. Because Mercedes felt it to mean the latter, they built the engine to work under the new directive of 0.9l, as the customer teams who have not used the engine yet will have to.***
- apparently the oil directive is being exaggerated and more relevant for qualifying. For qualifying, they use a thinner oil to achieve less wear. It's not about additives in the oils either. Apparently they can achieve up to 5hp like this. For the race, a thicker oil is used because it helps fuel efficiency due to better sealing of the cylinders and helps the longevity of the engine.
- The FIA has no means to measure the oil usage accurately. They use an oil stick, probably by measuring before the race and after to get a good indication of the usage. There is some tolerance for this reason. The FIA is working on better methods to measure it more accurately. By 2018, only 0.6 of oil will be allowed to be used and only one specification for the entire weekend.
http://www.auto-motor-und-sport.de/form ... 20994.html
(translated by best-effort).
*** (My speculation): Maybe Mercedes introduced specification 4 to have the ability to use the older models for other races under the old oil directive where it's necessary. No one dictates which engine has to be used at which venue. From what I understand, Mercedes has now 4 engine specifications in the pool. 3 under the 1.2l rule, the latest under 0.9l. Depending on usage, they could still use some of the older specs where it might be advantageous?