I've used the yellow Rector Seal # 5 which is supposed to be slow drying and soft set on my tank wagon.

When I redid my planter pipeline I used something that was recommended by a place that sells fertilizer supplies. So far no leaks.

It's called Rector Seal Tru Blue and is supposed to be vibration resistant.

I also checked at a place that sells fertilizer fittings and tanks and they were recommending the blue for applications where vibration could be a factor over say a stationary tank.

Here is what the Rectorseal folks show on their pages. 

Descriptions.

http://www.rectorseal.com/index.php/pipe-thread-sealants/

Chart

http://www.rectorseal.com/pipe-thread-sealant-chart/

(Rectorseal5.jpg)

(TrueBlue.jpg)

Rectorseal5.jpg (18KB - 91 downloads)

TrueBlue.jpg (27KB - 101 downloads)