What is the rainbow looking substance I see around my car in the rain ? I only see it in wet conditions

My car was leaking from fuel injectors a year ago, he glues 3 back in and replaced the seal on 1. Then it began leaking from leak off pipes, he put a cable tie round each one and now no visible leaks.

I am getting rather poor mpg though in my opinion from a car that should get 39.2 urban, 64 mpg extra urban and 52.3 combined.

Usually I only get 40 mpg with urban driving and several short motorway driving. so urban and extra urban I still usually only see 40 mpg. If I do almost full motorway trips with careful driving within speed limits I can see high 40's, low - high 50's. 

Lately I am seeing 38 mpg and that includes some motorway driving within the limits and with a light foot. 

When replacing glow plugs noticed some are very black 

Over recent months I notice a rainbow substance around my car in the rain and wet conditions, cannot see it when the weather is dry. There is an oil leak on the car I can see oil covering the sump and the suspension components but have been so busy fixing other things on the car I have not found out where it is leaking yet, I wash it away and shortly after the oil returns.

my question is, what do you think this substance is  that is rainbow effect substance coming from my car is ?  cheers