To address Haasman's original question, just use whatever weight the owner's manuals say to use. Both of my Benzes owner's manuals say that 10W-40 is the recommended "year-round" weight. The owner's manuals don't split between dino oil and synthetic they just list weights. I work with someone who has a 1985 Ford T-Bird Turbo Coupe 2.3l 4 cyl. He is the original owner and has always used 10W-40, per your request I won't mention the brand, every 3K. The car currently has 191,000 miles and has never been opened up. I agree with what another member said, that we can sometimes get carried away with the details about oil. The winning formula for oil is to change the oil often. The owner's manuals for pre FSS cars say to change the oil every 5k, under "normal" conditions. Mix in stop and go driving, towing, long periods of idling and short trips (real world "normal" driving), change the oil every 2500 miles. Simply because of that recommendation, I change the oil every 3K, since my driving and most peoples is a mix of the two.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section