Themed Float Ideas

Cavalcade-a group of horse drawn carriages or riders in a procession.
I see kiddos dressed like ponies, and some pulling wagons with kiddos in them, too cute!
Yarn manes and tails, everyone dressed in brown? Yellow felt for the "bridles and saddles"?
Not sure really what the point of the theme is when it comes to the "oil" part, but you could have the wagon(s) have an "oil spout" come up out of it. A black tube with some sparkly/shiney paper on it and some strips of the paper stapled to the top to 'spill' over and create a 'gusher'?

Perhaps you could borrow or make historical hats: derbies, top hats, etc. If police and fire depts have old-fashioned hats, that would be cool! I can't remember the exact title, but there's a really good Dr Seuss book about the hats of Bartholomew Huggins (I think) that would be fun.

Ask your community helpers: Firefighters, Police, Mailman, Doctor, Dentist, etc. to provide hats for the kids to wear.

Make Super large hats out of poster board, cut two holes in each (large enough for the child's arms), have them hold them up by placing their arms in through the holes, then they can still wave to the parade audience!