Comando:

mencoder -oac lavc -ovc lavc -of lavf -lavcopts aglobal=1:vglobal=1:vcodec=mpeg4:vbitrate=500:acodec=libfaac -af lavcresample=48000 -vf scale=368:208,harddup -lavfopts format=psp  -ofps 30000/1001 -o destino.MP4 origen.avi

Script para convertir todos los ficheros de un directorio:

ls | while read linea; do let l=`expr length "$linea"`-4;destino=`echo "$linea" | cut -c -$l`;mencoder -oac lavc -ovc lavc -of lavf -lavcopts aglobal=1:vglobal=1:vcodec=mpeg4:vbitrate=500:acodec=libfaac -af lavcresample=48000 -vf scale=368:208,harddup -lavfopts format=psp  -ofps 30000/1001 -o "$destino.mp4" "$linea"; done