How tight those bolts are will have no effect on the leak.
Have a good look at your turbo drain. This is the usual cause of this prob.
The other one is the rear turbo seal. If that is leaking it will cause smoke out of the exhaust on boost and when you pull up at the lights
If you dont have any of these probs id be looking at the drain. Is the eng oil level spot on. Ie not over full ?
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