I thought this might be of interest. I changed my oil when I bought my car two weeks ago. I must say the car didn't have that just serviced feeling you get after a oil change. If anything it felt rougher and the old oil looked pretty black. It got no better and the car felt very rough, almost like the cat was blocked or something. I then noticed oil weeping from the front and clutch judder.

The oil used was Comma 10/40 semi lite, it cost £15 for 5 litres which I know is cheap but it had the API ratings A3 B3 and so on. I changed it for Mobil 10/40 Semi and the difference is huge, its much smoother and the leaks seem to be diminishing. Looking back I had a main oil seal leak on a high mileage VW Passat td that started after using Comma oil. I could see it running down the back of the bell housing though the opening at the bottom. I had forgotten about this when I bought this oil recently.

I have no affiliation with any oil companies or suppliers, this is just my personal experience.

 

Wouldn't have thought oil would make quite that much of a detrimental effect. Having said that I never scrimp on oil or any fluid for that matter. I use Millers oil and to date have never noticed it dirty on any oil change on any of the Imprezas I've had.