Could be tranny fluid or oil still in the system. Now you said the oil light came on so I would suspect engine oil, did they replace all of the heater hoses as well as radiator hoses? If not they will burst (trust me) you also will experience mono valve problems down the road. I would flush the system with dawn after replacing (all) hoses. Radiator should have went to radiator shop to be professionally cleaned. We have done alot of 103,119,and 104 motors with this type of problem and learned from past experience. Just my 2 cents.