well, I always have an oil leak.... a car that leaks oil never rust...
my car is 24 years old so i expsect it to leak and mine always leaves
a little mark.... like a harley...

I find I have to add a quart or two every month. So its really good for the car getting fresh oil....

my car leaks around the turbo down spout, very common rubber gasket there that leaks....

Also my AC leaks some black stuff.... I need to repair this.....

Check your turbo, check the pipe coming out of the valve cover going to the breather... that brings oil gasses down to the turbo... there is a little o ring on the breather check it...check the turbo down spout....check the oil cooler,
check the oil pan... remember there two of them... upper and lower....
wash the bottom of the car.... the two most difficult oil leaks are the front seal and the rear seal.. and they are what usually leak on an old high mileage car....all you can do is use one of those oils with a seal conditioner and just hold on till rebuild time....(since your stp stuff slowed your oil leak it probably had seal conditioners)

If I were loosing a quart a week i would not worry about it...

but of course I am not one of these guys who use mercedes coolant...