nah.. its fine.. changing the oil regularly is more important than what you use as long as its not too shithouse.
I don't think they're as high as 85% mineral.. they're rated for turbo engines. It wouldn't say it on the bottle unless it was safe to do so. In fact there is probably **** all difference i reckon, other than the name and marketing associated with the expensive stuff.. but of course I'm not a chemist so couldn't determine that empirically..
Been using it for 18 months or so in mine, no problems. Doesn't seem to break down and go black that quickly, so i assume its doing its job reasonably well.