Over here it doesn't get very cold, but last night was a bit colder than usual, about 12F and I parked my 82 300D outside overnight. This morning it took me almost 10 minutes of repeated 30-second glow cycles to get it to start. It was probably 20F at the time. I've done all my fuel system and glow plug maintenance as well as valve adjustment and the car usually starts perfect when it's not too much below freezing. Currently I'm on 15W-40 Delvac 1300 and I'm wondering if anyone in this situation has seen a significant improvement after switching from 15W-40 to a lighter oil or if something else is causing the difficult starting.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL