I have a 20 liter can of Mobil Delvac 1 from which I have used 8 liters for an oil change in my car. I opened it about 3 months ago, take the oil I needed out of it and sealed it again with its original cap well tightened.

I measure my car's dipstick every week and smell it looking for diesel / coolant traces, every seems normal until today. When I took it out of the engine this "bacteria-full food" odor came out in big way. I tought it was head gasket or injectors seals, but then went to my storage and smelled the unused oil.... exactly the same odor. Do you think I can still use it? what is the origin of this odor? why the oil odor in the car changed so abruptely (I smell it on friday and it was OK)?

I also noticed engine oil kind of look thick, like it isa 15w40. The unused oil is ok, like water.... please help me.....