Fumed metal oxides (13 nm aluminum oxide particles and 20-25 nm zinc oxide particles), which are in the form of porous agglomerates of nanoparticles, are effective as thermally conductive solid components in thermal pastes. They are as effective as carbon black, but are advantageous in their electrical non-conductivity. Without fuming, the oxides are less effective. By coating with silane, which decreases the viscosity of the paste, they are even more effective. The organic vehicle (polyol esters) and solid component content (2.4-4.0 vol.%) are chosen to attain conformability and spreadability. The use of either 4.0 vol.% silane coated fumed zinc oxide or 2.4 vol.% silane coated alumina gives thermal pastes that are more effective than commercial thermal pastes (Ceramique and Shin-Etsu). Fumed zinc oxide is superior to non-fumed zinc oxide in improving the thermal stability. Silane coating of the fumed zinc oxide further improves the thermal stability. Fumed alumina does not affect the thermal stability, but silane coated fumed alumina improves the thermal stability. Though silane coated fumed zinc oxide is superior to silane coated alumina in enhancing the thermal stability, it is slightly inferior in the phase separation tendency.