If it is progressively getting worse then turbo oil seals would be the first culprit especially if there is excessive play in the shaft.
If the turbo is sound then I had a leaking valve stem seal but I've never heard of that issue on the L series before but it is also a possibility.
If the connection from number 1 injector is damaged then this can also cause white smoke apparently.
My first port of call would be the turbo
