Little Dog
A2OC Donor
The tandem pump delivers more fuel than the injectors need along with any oil contamination. What the injectors don't use flows back to the tank taking the oil with it.I can see how oil could pass into the fuel line, but not how it would then flow to the tank. Surely it would mix with the fuel, and end up being burnt, may upset injectors, depending on the ratio of oil to fuel.
I think the contamination in the tank is more likely to be be paraffin wax thickening and settling out.
By experience, the filters in diesel forecourt pumps need changing more often than the petrol ones.
Diesel quality can vary too. Blend varies summer to winter, with wax dose reduced in cold weather.
Mac.
I'm thinking much the same though; that contaminated or inappropriate quality fuel has been put in the tank.