Langar fuel and timing re Zorkul
I didn't do Zorkul, but I have two bits of info for you.
I got fuel in Langar last July. Langar is a sparse and scattered settlement. The place for fuel is couple of kms before the blue sign-posted homestay (on the right as you ride East and the only one I remember seeing). It's in a lone white-washed building/shop on the left heading East – ask around since there are no signs. From barrels, as if often the case in Tajikistan. But like MCole says, someone will help you if you're desperate.
And re Zorkul timing. I would aim to do it from mid-July onwards/ August. By this time the track should be dry. In Dushanbe I met an overlander with a Landcruiser. He had taken the track from Mughab to Zorkul and onto Hargushe Pass in June, but had got bogged down in mud somewhere near the lake and had to be toed out. All of which he loved. Stunning scenery he said.
Have a great time,