Mobil 1 Tri-Synthetic Formula 15W-50 is especially suitable for high-performance cars, particularly hot-running or heavily loaded vehicles and special high-tech imports."

Flash Point, ASTM D 92, °C (°F):
0w-30
238 (460)

5w-30
235 (455)

10w-30
243 (470)

15w-50
245 (473)

Flash Point, ASTM D 93, °C (°F):
0w-30
211 (412)

5w-30
210(410)

10w-30
221 (430)

15w-50
223 (433)

5W-30 has the lowest flash point of them all(bad). 15w-50 has the highest(good). Seems to me like 0w-30 would be better to use before 5w-30

Here is Mobil's comparsion of the 4 types:
Mobil 1 Tri-Synthetic Formula 0W-30, 5W-30, 10W-30:
Flow at -54°C (-65°F); pump at temperatures lower than similar SAE viscosity-grade conventional oils. Save gasoline, exceed newest Energy Conserving Requirements.

Mobil 1 Tri-Synthetic Formula 15W-50:
Flows at -48°C (-55°F), pumps at temperatures as low as many conventional SAE 5W-30 oils. Provides superior high-temperature protection for high-performance and high-revving cars.

(Looks like I can run 15W-50 all year round)

All Mobil 1 Tri-Synthetic Series products:
Provide outstanding protection of gasoline, diesel and turbocharged car and light truck engines.
Provide excellent engine protection during high-temperature operation.
Extend engine life by reducing wear.
Allow faster cold starts.
Help prevent oil breakdown; keep engines cleaner.
Are compatible with conventional oils.
Provide maximum reserve capacity in a motor oil.
Reduce oil consumption.