Is it the strut mechanism or the pivots?
I'd suspect the latter and pretty much anything except WD40 is OK on them. Thin oil, or silicone spray you mentioned. Best way to get it in is to prise out the small retainer clip at one end, pull the strut off the pivot and lubricate. Repeat on all 4 pivots. Remember to support the tailgate as it will only have one strut connected.
As the struts have oil in there should be nothing to lube, just ensure the rod are clean, however your silicone spray won't harm