I got bad glow plugs in my 300SD but was able to start in cold anyway...until today, minus 5F in Chicago and windy!!

15W40 oil in my engine not changed yet to 5-40 so not so optimal for the cold start. There was Howe's in my tank for couple hundred miles which makes huge difference, now I fill the tank with bottle of RED LINE RL-3 winterized.
Too far to pull extension cord to my car for hair dryer so I heat the air intake using plumbers butane torch for some time - starts like it's summer.
I am not rushing so much to change my plugs anymore...