Given that the recommended change interval for mineral-based oil is 7500 or six months, a full synthetic should be okay for 10K, but why not just use conventional oil and change the oil and fiter every every 7500, which would be three months. Assuming you do your own oil/filter changes, you should save enough in oil cost to make it worth your time. If the engine has low oil consumption, consider a HD diesel engine oil, which will benefit the engine due to its better additive package, but the additive combustion byproducts can degrade catalysts, so only use CI-4 oil if consumption is low.

Highway miles are easier on the oil than stop-can-go traffic, because the oil temperature will be lower cruising at freeway speed than urban traffic and you have less hours on the oil and fewer cold starts the higher your average speed over the interval. Back when my 190 was a daily driver with a mix of city and highway miles I changed the oil and filter every 5K, which was about three times a year.

Now that the 190 is my "winter car" and only gets used on average once or twice a week accumulating 1000-3000 miles over the six months it's in use I change the oil once per year - just before I store it for the summer.

BTW, I will be swapping the '91 MR2 for the Merc in the next week or two. It's always nice to rediscover the pleasant driving charactericts of the 190. Swapping cars every six months keeps them from getting boring, especially two cars with such different driving characteristics. Just got back from a 1000 mile road trip to Nevada City over Thanksgiving over mostly two-lane roads. Highway 33 over Pine Mountain Summit is almost as good as a day at the track!

Duke