Synthetic oil in 1988 560SEC?

[lws1]

My 560SEC is due for an oil change and I was thinking about synthetic. If so, what kind and weight for my baby that has 161,000 miles, timing chain was changed about 1,000 miles ago and I drive her once a week. Also, I live in Miami, FL. If I can switch to synthetic, how often do I need to change the oil because I don't put the miles on her? Obviously you can tell I am not a mechanic and haven't changed oil since I was a teenager on the farm. I was going to take in to to a new mechanic that is closer to my house but thought I'd bring my own oil and filter and have them do the work and look her over. Thanks, LWS1