You may be familiar with the normal mode of diesels whereby you start the engine as soon as the light goes out.

Now, this may be true for a Ford or GM, and it may have also been true for your M/B when it was brand new, but, don't go by the light anymore with an engine that has 200K on the clock.

I let the SD glow for about 15 seconds when the temps are above freezing. Down below freezing, I let it glow for 30 seconds. The light has turned itself off a long time ago. The benefit is reduced cranking time and an immediate smoother idle when it starts.