Chain store media can be a bit of a mix, fake codes, or lower rated media.
Or to take another example, Poundland...
Connect-it DVD+R = fake YUDEN000 T03 code (but error rate was ok)
Signalex DVD-R = CMC MAG AM3 (quite likely real) - claimed to be 8x, but code is 16x - now my strong recollection, and results from some Memorex CMC MAG AM3, are that it's a very hit or miss media - some drives may like it at full speed, but very few do.
I may be maligning the Memorex somewhat, as it's a part used pack that was in a charity shop, and may have been stored badly.
Anyway, Supermarket, or even big retail name brands, are no guarantee that what's inside will be decent, so if you can source some respected name media, it's probably better.
Are you sure on -R though, not sure if it still applies, but some drives tend to be better with -R, others with +R - always used to depend which camp the makers were originally in.