Bob on Paul..
Dave.. I have a PX similar to you and I change the oil once a year, used 2 plugs in 12 months, I run a few drops of oil down inside the cables and check me bulbs twice this year to see if they are blacking over so as not to get caught out in the dark. Best thing I did to it this year was invest in two Conti Navigator tyres which do seem to stick to the road well.
Another good move was to wax oil the underside of the scooter and also the front mudguard. Did mine from new using only one spray tin @£9.99 and it does help protect the bodywork from the elements. At the end of the year I take to the local garage and low preasure wash the underside with warm water, dry then check the wax oil. At present its all still on and seems to protect very well. If you do a wax oil give the underside of front mud guard a good going over as its the first thing to rust up at the seam. Water seems to find its way up from under and into seam then lifting the paint.