The seller has overwhelmingly positive feedback, and the only negative ones have been about stock availability and refunds, not product quality.
Personally I would just use the stuff, though you could always send off a sample
to have it analysed. Except that would wipe out whatever you saved on the cost of the oil in the first place.
I've never used a flush, on any of my cars. Not even when changing the type from semi to full.
I could be wrong, but it seems to me that a flush is really only necessary when the oil has been allowed to stay in the sump for too long and there's sulphur compounds, soot and water in there.
Just make sure you get her good and hot before you open the plug. And learn to push the plug in until it's off the threads, then pull it away sharpish because burning oil
sucks. Even through gloves.