Here`s a picture of that gritty looking stuff that was at the bottom of the jug of Valvoline I got at Walmart yesterday. You can see it mostly towards the lower right side of the bottom.
I used to get similar stuff years ago in the white Castrol bottles. Don't know if it is additives that fell out of solution or dirt contamination. Shouldn't be there in any case. Don't recall seeing it in other oils, but wonder how much the white bottle makes seeing it so easy and how difficult it would be to see if it was there in something like the black bottles of Havoline?