A treated silica, for use in a silicone grease composition (see Division C3), is prepared by merely mixing a finely divided silica (fume, xerogel or aerogel) with boric acid, trimethoxyboroxine (methylmetaborate) or a trialkylborate in which the alkyl radicals contain from 1 to 5 C atoms. Preferably the mixture is heated at 100 DEG to 250 DEG C. for 1 to 6 hours. The boron compound is preferably used in amount to provide from 0.001 to 1 pt. (by wt.) of B per 5 to 17 pts. of silicon. In Example (3) (H) 10 pts. of silica aerogel were mixed with 0.75 pt. of trimethoxyboroxine (0.135 pt. B) and the resulting mixture was heated at 200 DEG C. for 5 hours.ALSO:A stabilized grease composition comprises (1) a major amount of a silicone oil, (2) a minor amount of a finely divided silica as thickener, and (3) a stabilizing amount of boric acid, trimethoxyboroxine (methylmetaborate) or a trialkyl borate in which the alkyl radicals contain from 1 to 5 C atoms. The greases are made either by blending the various ingredients together, or by pre-treating the silica with the boron compound (see Division C1) and blending the resulting product with the siloxane. The mechanical stability of the grease is further improved by heating the grease at 100 DEG to 250 DEG C. for 1 to 6 hours. The siloxanes used may contain alkyl, alkenyl, aryl or halo-hydrocarbon radicals. The silicas (fume, exerogel or aerogel) are used in amount (by weight), e.g. of from 5 to 17 parts per 100 parts of siloxane. The boron compounds are preferably used in amount to provide from 0.001 to 1 pt. of B per 100 pts. of siloxane. If desired, some of the silica may be replaced by silica which has been treated with octamethylcyclotetrasiloxane, as described in Specification 806,563. Other fillers and stabilizers such as pentaerythritol and high molecular weight polyalkylene glycols and glycol ethers, may also be added. Examples are given. Specifications 709,920, 998,657 and U.S.A. Specifications 2,428,608, 2,469,890 and 2,469,888 also are referred to.