Putting a different brand of oil in is a "modification".
You have to draw the line and be sensible.
Yes - I agree if you re-chip the ECU (in software or hardware) or stick a nitrous kit on or get the engine over-bored, or even shove a bigger engine in - these are "modifications" that an insurer has a legitimate reason to want to know about.
Putting a make of tyre on that meets the basic spec and happens to be more suited for the prevailing conditions is NOT a modification.
Yes, winter tyres seem to be lower speed ratings, but when the rating is
130mph and a) I cannot do that in the UK, and b) my car could not even do that on an autobahn and even if it could, I'd shit my pants - it's not really relevant :)