Are 使用 and 施用 interchangable? In my on-going quest to understand the difference between initially indentical-looking words, I hereby bring you: 使用 vs. 施用! Both seem to mean 'to apply something' and it appears to be acceptable to both 使用化肥 and 施用化肥. 
Maybe I have hit a genuine synonym this time?! If history is anything to go by, probably not, but it's a fun game, eh? :-)
Hashamyim
施用 = use; apply; implement (usually to a target)

使用 = use; operate (with or without target)


When someone "施用武力" or "施用毒氣"，  there are always specific targets the action is applied to 


Example: 對示威者施用武力; 對村民施用毒氣 use force against demonstrators; use gas on the villagers (you can replace 施用 with 使用 in these two examples) 


When someone "使用工具" or "使用武器" there may or may not be a specific target the action is apply to.


Example: 祇有小數動物會使用工具/武器 Only a few kinds of animal know how to use tools/weapons ( you cannot replace 使用 with 施用 in this example because there is no specific target the action is apply to)