[Kevan]

Ok, oiled tyres with 3 in one, how often should I re do them ?  

With NSR tyres (not the Extremes, don't get oil near those...in fact don't buy those, they ball-up just looking at them) you can get away with once per day for a week then they'll be as good as they get.

And how can I find correct tyre size for a wheel,  it’s 15.8 x 8.2.  Thanks

If I have a wheel that size I'd fit my favourite tyres that will fit, so 9mm wide and 20mm diameter.  Rubber stretches, don't get hung up on getting tyre sizes spot-on (unless you're going down the silicone tyres route), I recently stretched a pair of Thunderslot tyres 10.8x19mm over a pair of 17mm dia wheels.