5 minute flush? Full synthetic oil?

[William73]

I'm about to do the first oil change in the '87 300E with 156k I just bought.
I like to use a can of 5 minute flush before I change the oil. Then again, I usually do this in an old International, not a Mercedes-Benz.
Would it be a bad idea to do this?
Also, I'm thinking of putting in full synthetic oil. Any suggestions on which oil I should use? Is there a good oil additive I should put in it?
I'm also planning on changing the tranny oil and filter. Is there something I should do before I drain the tranny, like a 5 minute flush? I would like to have it nice and clean when I put the new filter in.
I'm looking to get long life and good economy out of this car.
Thanks for any help you may have for me.