The critical maintenance on these engines seems to be the need to keep a close eye on the tappet clearance on the exhaust valves.. This tends to get neglected because they are hard to get at, and if allowed to run with inadequate clearance is likely to result in burnt valves, and possibly the need to replace valve seats.
The cooling system needs to be kept in good condition to avoid overheating. There is a pipe inside the block that modifies the coolant flow to ensure that the exhaust valve seats are not overheated. If coolant is allowed to become corrosive, this can corrode unseen and lead to issues with No6 exhaust valve.
There is no reason not to use a synthetic or semi-synthetic oil, but the main advantage of these is extended service intervals, and I would definitely not advise extending the service interval, even with these oils, so it could get expensive. (This is a 1940s design engine, and the (by modern standards) short service interval is needed to deal with combustion products and possible fuel dilution as the mixture is not as well controlled as a modern engine.)
Although this engine usually pulls well at low rpm, it is better not to allow it to do so - ultimately, this is a car engine, not a truck engine, and is quite happy at high rpm. Use the gears!
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