What kind of oil to use in 300E???...

[Agron80]

I got an oil change today, and my mechanic put in 10W-40 (Castrol GTX), I've been using Castrol GTX 10W-30, for a while, and now the mechanic went the the change.. My question is--what viscosity should I use? I know this topic has come up milions of times, and I know it has to do with the climate conditions, and your driving habits. Im not a rough driver, and I drive mostly highway miles back and forth from school. Here in New York, we have cold winters, and hot & humid summers. Which viscosity should I use in the winter/summer??

P.S. I do not use synthetic

Thanks in advance...

Agron
1989 300E (105k miles)