Futura a bit lethargic low down.

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Evening all

Looking for a wee bit of advice here before I go getting a full diagnostic done on my 98AU Futura.She's done 255,600km and still goes great.Starts first time every time!I recently gave her a full service with new filters,oil and plugs.Went for a trip up north and she went sweet as-mid range and top end power is awesome.Even at 120km per hour I could kick down to overtake and she takes off!
But what I have noticed lately(before service also),is a bit of lethargic performance in the low rev range-your general around town driving.
At service I went to a slightly heavier grade oil,but I doubt this is the problem??
Plugs and leads are fine,compression is spot on,no excess fuel comsumption or weird flat spots or the like.
So,after reading other similar posts and replies should I be looking at the O2 sensor and cat first? I wouldn't know the O2 sensor if I fell over it,and don't know what I should be looking for on the cat.It certainly is not rusted or damaged on the outside,but thats not what counts is it?Exhaust note sounds pretty normal.
Any advice appreciated.