Yesterday I put synthetic oil in my wife's 2000 Golf. We went up to town (10min) and I'd swear the engine seemed smother.
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Eric--- Both our everyday vehicles use synthetic oils. One of the vehicles can use pure synthetic or synthetic blend, the other requires 0-20 pure synthetic.
Like all "machines," I think it's a hand-in-hand situation. Engines can be made more efficient without sacrificing power, or simply made more efficient, by design advancement that are in turn made possible by new technology developments, improved metallurgy, and so forth. But in order for these advancements to work, there need to be advancements in lubrication, heat dispersal, and so forth.
So the lubricant folks come up with products that allow the new engine designs and technologies to work. This in turn, leads to even newer design ideas and technologies, which in turn may require even better lubrication properties.
And so the upward spiral continues.
The fact that the new lubricants do a superior job of lubricating under conditions of tighter clearances, higher heat, and so on, makes them appealing to the operators of older engines. "If XYZ Oil is so fabulous at lubricating an F1 Ferrari engine at 15,000 rpm, or a twin-turbo, all-aluminum, high-output truck engine, then it oughta be great in my old Ford Lehman."
Well, maybe it will, maybe it won't as you've pointed out in past posts and as some studies have shown.
But what's more important is that in it's heyday (1950s in this case) that old Ford diesel wasn't old. In fact, it represented the then-latest in automotive diesel technology. And the reason it was designed the way it was is because the lubricants and coolants of the day permitted it. Just like the 0-20 synthetic lubricant available today made it possible for Subaru to do whatever clever things they did in their latest engine that went into my wife's new vehicle.
Which means that if one runs a Ford Dorset diesel aka Ford Lehman 120 in their boat and they run it on the oil that the engine was designed to run on, and they adhere to the manufacturer's recommended service intervals and operating parameters, they will get the very long service life that Ford of England had in mind when they designed the thing in the first place.
Sure, a synthetic might reduce friction a wee bit inside the engine. It also might start the engine leaking oil from places one never thoght an engine could leak from. You won't know until you try it.
But my point is, why bother? If you use the oil that Ford said in 1950-something shoudl be used in their Dorset diesel, the thing is going to run for many, many thousands of hours with no problems whatsoever as far as lubrication is concerned. As others have said here, the engine will crap out to the point of needing an overhaul way before lubrication becomes a problem (unless you let the engine run out of it).
So my philosophy is run an engine--- any engine--- on the oil it was designed to be run on and you'll never go wrong.