Toto Wolff has explained why Mercedes could not supply the latest upgrade of its power unit to its customer teams for the final few races.

Mercedes supplies power units to Williams, Force India and Lotus -- as well as its works team -- and at the Russian Grand Prix all three customers used their fourth and final power unit of the season (using a fifth set of power unit components will incur a grid penalty). But rather than receive the specification used in the works Mercedes cars since Monza, the customer teams received the same specification -- in terms of performance updates -- they have used since the start of the season.

Mercedes spent its seven remaining performance tokens ahead of Monza, but Wolff said the engine department only has the resources to apply those changes to the two works cars and not the customers.

"The [upgraded] engine is very much an R&D engine," he explained. "We decided on trying out the direction that we believe could be beneficial for next year but we are not 100% sure.

"In order to do that exercise you can only concentrate part wise and logistically on the two engines. When we had the problem with Nico's engine [at Monza] we weren't even sure we had the spare parts to give him another engine. This is why we decided to do it."

Wolff insists the spend of seven tokens ahead of Monza did not dramatically improve the performance of the power unit, but has opened up potential development opportunities for 2016.

"From phase three to phase four [the Monza upgrade], the main thing it is not a performance step up. Of course people always say that engine must be much better, but that is not the case and it is just a development direction for next year. We can probably run it a bit harder for a bit longer, but it is not the miracle step up and we just can't supply it logistically."