'86 Westy

I got into my van after it has 'sorta' been sitting for 3.5 months.  I say
sorta because it has been started for extremely short periods of time by
the body shop it was in, just to move it around the shop.

As the engine was warming and I was running at speed, plumes of white
smoke came from the exhaust, especially when downshifting.

My first thought was coolant leak into a cylinder, but the smoke minimized
(but did not go away completely) as the engine warmed up to operating
temperature.  After cool down the coolant level in the expansion tank had
not moved appreciably.

The van was stored with very little gas in it and I had just filled it
up....could it be water in the gas or do I reallyy have a headgasket leak?

Any ideas?

Thanks
Gregor