Chris, may be worth looking at one or two other things not overfill related;
Sounds to me that being laid up for a few months is a clue. Normal for the M64 engine after decades and 000's of miles service to have one or two very small leaks with the odd spot on the garage floor (nothing to worry about or to listen to rebuild stories about either). With regular driving (say once a week or so up to temp) any small drips burn off without being noticed. However, if laid up for a while some oil drops 'soak' in to hidden areas such as on top of the secondary muffler (or G pipe if you use that instead). This area typically from the oil filter housing in/out pipes behind the wing. Another area may be oil/fluid soaked, in or resting on top of, the heat exchangers (towards the rear axle end). This area from potential very small power steering fluid or engine oil filling spills/leaks that have graivited towords the rear of the engine and out through the tinware on to the exchanger.
A couple of years ago I had exactly the same white smoke issue as you but knew oil level was fine. Realised I hadn't taken it out for a spin for over a month. Wheels off and straightaway noticed oil staining/dampness on the filter feed pipe at its' lowest part, ran a finger over the top of the secondary muffler, again some oil staining. Muffler came off then could see that further back on the muffler mounting bracket was more oil staining, but this time from above (engine tinware gap corner of engine where steering fluid and engine oil spillage from topping up collects). Took this as an early Spring opportunity to strip oil pipework off, remove filter housing, make good and redress all compression surfaces and refit. Power steering pipe replaced (reservoir to pump) as this can be prone to old wear cracks and leaks round the out-of-sight union into the pump (cheap OPC pipe).
If white smoke then it's usually oil burning off at a different (hotter) temp. If an engine issue then maybe not so white and would stay with you most running temps. If also coupled with rough idling, or less than ideal throttle response low down, then check the ISV for oil residue and clean out with carb cleaner and leave to dry. If you definitely know you had a quite a mega overfilling then maybe also remove the MAF sensor/meter and take off the throttle housing rubber 'manifold' ("air box" in the Katalog) and give that a good clean out of all oil residue (MAF too, with some care).
Oil consumption for the 964 was high out the factory (1.5L or 2.6 pints every 620 miles!), so don't be put off by needing to check level frequently and topping up if you have to.
Plenty of waffle but hopefully useful..
Marcus
