In contrast, the Bf 109 was a superb dogfighter and above was in its element. In the spring, 1943, I/JG 2 and II/JG 26 were flying Bf 109s and Fw 190s. Operationally at "gruppe" level this was not efficient and it was decided for these units to retain their Fw 190s; and did so until the end of the war. The Bf 109 and Fw 190 were used to complement each other in the coming battles. The Fw 190s armament, considered effective against all enemies, was used against bombers more frequently, while the high-flying Bf 109s engaged escorting fighters. The Bf 109G-4 was “up-gunned” as well to the Bf 109G-6, with two MG 131 machine guns replacing the MG 17, and supplementing the MG 151/20 cannon in the nose. The MW 50 (water-methanol) additive increased lower altitude performance but the increase in weight reduced manoeuvrability. German pilots were critical of the Bf 109s fragility, but praised the Fw 190s strong construction; the latter type remained the preference among western theatre pilots.