[Tatsfield]

Really hopefully the problem isn't a head gasket but the garage will have a coolant tester to detect exhaust products and that would be a sure proof.

If it is to do with the regeneration of the DPF, the problem stems from Volvo adopting a DPF system which burns out the filtered carbon particles by having the fuel injectors inject fuel into the cylinders on the exhaust stroke, sucking unburnt diesel into the exhaust where it burns in the filter housing. Unfortunately some of the unburnt fuel in the cylinders is scavenged by the piston rings and ends up in the sump. The piston scavenging is designed to deal with lubricating oil in the cylinders and that causes no problems being put back into the sump from whence it came, but diesel fuel shouldn't be in the sump and after a lot of filter regeneration the sump level can rise 30% which is why Volvo rechristened the 75% mark as "Full" leaving 25% space for misplaced diesel fuel. This dilutes the lubricating oil, thinning it so that more of it gets past the piston rings and is burnt in the cylinders on the compression stroke. Whether that causes pale smoke in the exhaust I do not know but I do worry about what it does to the lubricating properties of the oil in the sump! Volvo don't like to talk about this simplistic workaround of a basic design problem in their DPF system. I'm not surprised.