Could it be the oily stains are just not visible on light clothing, or the loads are run at different temperatures, or some other variation in way of explanation for the different results? Otherwise it implies there is something about your dark clothing that is the source, which I find difficult to believe.
I really think the source is the machine itself. We once had a well used machine that started doing the exact same thing. It happened to all clothing. We never really determined the cause. We eventually gave up trying to stop the stains. As soon as we used a different machine, we never had this kind of staining again.
The only thing I can imagine is oil from the machine's mechanicals somehow found its way into the wash water. This could possibly be through a perished seal at the pump, or maybe solenoid valves, or maybe it was an external source, once introduced difficult to get rid of. If we could be sure of the source, a replacement part could solve the problem. But I'm just guessing and am not confident that replacing some parts would solve the problem.
Unless you hear of plausible fix, I would consider replacing the machine straight away. Blindly replacing parts gets expensive, that money could better go towards a replacement machine. Washers are not inexpensive, but neither is clothing. It doesn't take a lot of ruined clothing to start costing more than a replacement machine. Do consider any other plausible fixes first, this is the action of last resort.
I'm sure it seems extreme to suggest a complete replacement based on a brief Internet exchange, but I've been in your position. The thought of how many clothes we ruined still sickens me. I would never suggest this if it hadn't happened to me personally.
First, let me say, you are far from the first person to have smell problems with washers, especially front-loaded ones. This has been the topic of many conversations resulting from home inspections and with concerned customers. I have had followup consults with factory reps, appliance repair specialists and the like.
The most common problem seems to be with the rubber sealing rings at the door. Water sits in the folds, mold grows in the folds and is untouched by the "washer cleaners" that are currently on the market. Wonder why they are so popular? some folks have been able to manually clean these areas with bleach and many rags. Others have had to have it replaced then keep it clean before the mold takes hold again.
Any closed reservoir can harbor mold and usually does. If you cannot access them to clean the mold out, that part usually has to be replaced. The other problem is if small amounts of water sit in the bottom of the unit and don't get expelled. this is easily treated with flushing cycles with bleach or cleaners.
Before you completely give up on the unit, see a Bosch dealer with a good service reputation. Several models from several manufacturers have been recalled or service bulletins issued for exactly the problems you are having.
Best Answer
Putting the detergent into the dispenser drawer allows the washing machine to make sure that the detergent is mixed with water before starting the cycle.
If you just throw the detergent into the drum it can get caught up in the clothing and some of it can end up never getting fully distributed throughout the wash water.
This is more of a problem with dry powdered detergent but can also be an issue using the concentrated HE type liquid detergent with the newest washers that purport to be high efficiency through the use of minimal water.
In the olden days I can remember my mother mixing the powered detergent in water before introducing it into the old style wringer washing machine she used in our basement. She did that for the same reasons as the washers at the time only had agitate on/off and no automatically controlled water fill and spin cycles.