^ yeah that's the catch. Before doing a flush, I'd do several short OCI's and get it good and hot for 1/2 hour driving spirited before each oil change.
I wouldn't do a BG "service" at all though, even seafoam seems pricey to me considering the ingredients. I'd just dump a few ounces of naphtha (which is what many lighter fluids are) and a few more ounces of diesel fuel in, let it idle for 20 minutes, and then drop the oil pan to see the carnage. I would not do a flush without dropping the pan to clean it out and inspect the pickup. Then again, better to just not buy something that has had insufficient oil changes. If even the oil isn't changed often enough, anything more than that being done often enough is pretty unlikely. YMMV.