Black smoke from tailpipe of BMW 320D

Hello, i have a BMW 320D (2003), which seems to give out a lot of gray smoke from the tail pipe, the cool-lent levels are ok it does not over heat, cant see any pools of oil on the road have check dipstick for oil its normal (oil indicator on dash does NOT light up). it has got the power to pull (accelerate fast) but i have noticed it to be weaker then what it was few months ago, smoke does smell heavy diesel/oil, i drove it this morning with very little smoke but other times it leaves a big cloud behind it seems a bit intermittent, motor oil on dipstick is almost black, any ideas of what may be the cause. Many thanks in advance.