Chain oil can easily contaminate your rear braking system. Perhaps time is the best way to get rid of it. Alcohol?

How much do your brake levers move? If they're bottoming out before stopping the bike, then you need levers with more pull or callipers with less pull, or an entire system with less flex, or tighter adjustment to the brake callipers.

I think some of the more modern levers such as the Origin8 Classique, TRP RRL have a moderate amount of pull. The Tektro R340 may be similar. The Tektro R520 levers have a LOT of pull.