Anti stiction oil, basically 15w motorcycle fork oil, the factory techs used Maxima Fork oil.
With all slides in, all jacks up connected to your truck, use a turkey baster to remove 1 qt of the oil from the reservoir & replace with the fork oil then extend/retract the slides/jacks a few times to mix in the new oil, retract all, check fluid level it should be 1/4-1/2" from the fill opening.
In the future top off the reservoir with the fork oil when needed.
Danny & Linda
Former '13 FB owner