The the patentee in the decision "Reifenabdichtmittel" had amended a limitiation according to which an adhesive for sealing tyres "comprises" a number of ingredience to a limitation that the adhesive "consists of" the same ingredients, i.e. that no other ingredients were used.
The BGH fond that, as a rule such an amendment is not allowable because it introduces the new technical feature of the "conclusiveness" of the recipe and that furhter incentives from the specification are needed to make such an amendment allowable. The specification mentioned indeed an embodiment with no furhter ingredients (such that the amendment would presumably have been allowable under the EPC) but did not explicitly mention this feature. Rather, other portions of the specification indicated that fillier materials could be added optionally and this option was not explicitly excluded for the embodiment under discusstion.
In the judgement of the BGH, the allowability of such an amendment would require indications that the feature of consisting of the mentioned ingredients only has particular advantages or is otherwise desireable.
It is interesting to compare this decision with T 390/08, where the "consisting of" in claim 1 in combination with a dependent claim mentioning further ingredients (or more precisely, the introduction of a further element in the opposition procedure) led to a clarity objection. There appears to be a tendency to require some explicit "singeling out" of specific combination of features to be claimed, which may be supported by mentioning particular advantages of the specific combinations.
For the practice, this means that when mentioning the advantages of optional additional features for supporting their inventiveness during the prosecution, it is wise to also mention an advantage of the embodiment without these features. In the case at issue, it would probably have been sufficcient to say that the adhesive is cheaper or easier to brew if no filler materials are used.