Yeah I like the idea of criminals being able to fit some sort of spike strip of their own to a vehicle. Oil slick would be really cool because I could make the vehicle spin a bit when going over it, but I can't really think of a good way to show an oil slick in-game. If memory serves right, I believe the is an 'oil spill' texture, I'll have to look into that.
https://dev.prineside.com/en/gtasa_samp_model_id/model/19898-OilFloorStain1/As a bonus, it looks like a dick. Perfect. Not sure how big it is in-game though. Creating it on a slope will also be a challenge as my maths is shitty, but spikes already do this to some degree so hopefully I can just base it on that (if not just copy it directly).
My only concern is that it's going to be pretty hard for cops to see. Spike strips are really obvious when placed so therefore avoidable. With an oil spill you'd probably just assume it's part of the road by default..