Have to admit having to put a tankful of 95 in my 370 during the the shortage a few weeks ago, but it was that or walking! Can't say I noticed any measurable difference at all. In fact got the same mileage out of it that I normally get on a tank full ofsuper.
I wouldn't do it normally, it was an emergency, so please don't hate me!
I can't see it causing any damage to the engine long term, just might be a bit down on power. If the 98 you put in is old, it's probably only as good as 95 anyway with the way petrol degrades.