It's a basement-era fic. Spike becomes a bit infatuated with Xander. There is a scene in an appliance store where Xander works and he uses a type of machine oil as lube. (The things that stick in one's brain!)
Later in the fic, Spike finds an old collectable comic book in great condition to give to Xander. Spike brings it to the basement and somehow gets confused since Xander isn't there and gets the impression that Xander is dead. Spike becomes very upset.
Spike finds out that Xander has simply moved and gets frustrated with him. Xander eventually catches a clue and vamp-proofs the apartment, gets ashtrays and black mugs, etc. He gives Spike his own set of keys and tells all the Scoobies that they are in a relationship.
I've looked through the Spanderfiles but I don't think it's there. Please help!