Smokey 4.2 TD Cruiser 80 Series.

Hi Guys,
Just wondering what would cause this smoke.
I've got a 91 TD GXL 80 series manual cruiser.She's got 286500 k's on her now, goes a treat. But she blows smoke. Has done for a while now. I've noticed when I travel away from the sun late evening. It's whitish in colour. A mate has followed me and said that it's not dark like oil smoke it's rather white and fine not heavy and black. After doing some heavy beach work at high tide in the soft sand the smoke completely disappears. He said it does not smell like oil smoke. When cold it is more noticeable. It does appear to burn oil under normal driving conditions but under heavey slow going it does, not much though. Any thoughts would be appreciated.
Another query I have is when I bought her 3 years ago she came with 31x10.5x15 Duellers on her. She does about 2500rpm at a 100k, is this normal rpm for that speed? I ask cause I find myself wanting to reach for another gear when doing 100kph? I was thinking that the next set of tyres that goes on her would be 33's to lower the revs that she does just a tad.
Do those revs sound about right for doing a 100k?.
I was wondering if the two are related.

Hairs.
The 50,50,90 rule
When you have a 50,50 choice,
90% of the time you'll choose wrong.
Works for me every time :)