Sorry to bring up another oil thread.

This forum and the Mobil 1 website have tons of information on synthetics. I'd like to take the plunge, but I'm not sure what viscosity/weight/grade of Mobil 1 to use. My Toyota takes 10W-30 so I'd use Mobil 1 10W-30. My 91 300SE takes 20W-50 year round. The closest match is 15W-50. Should I use another viscosity/weight/grade in the San Jose, CA area? The 0W sounds intriguing, but will I regret it when the temps hit the hundreds?

Thanks,
Sixto
91 300SE
138K miles
1 qt every 5K miles