Marin,
They just want you to use the thin low vis stuff so their Subaru's have the best EPA millage numbers possible.
Don't think that's the reason, Eric. Long time ago Chevy made a 40th Anniversary edition of the Corvette. Our neighbor at the time bought one. Nobody gave a hoot in hell about the fuel mileage of the thing. But the manual specified a very thin synthetic oil because of the engine's very tight tolerances and the warranty was void instantly if you put anything else in it. It had nothing to do with EPA ratings. And I think the deal is that if you have a very tight tolerance engine and use the oil specified for it, it won't wear down to loser tolerances, at least not for a long, long time.
I agree that running old engines on new-technology lubricants may not accomplish anything much. But the reverse is not true. Running new technology engines on "old-technology" oils can be very damaging or at least detrimental to their efficiency and performance.
The lubricants used in today's GE, Rolls, and Pratt turbofans is not the same stuff that was used in the jet engines of the 50s and 60s. Today's engines are built with very tight tolerances where these kinds of tolerances can improve the efficiency and performance of the engines. GE doesn't say, "Well, we'll run them on the stuff they used in the 50s because the tolerances in our engines will eventually loosen up and the new stuff won't make any difference." Instead they do everything they can to preserve the efficiency and performance of the engines-- which cost millions of dollars apiece so the operators want them to perform as advertised for a long, long time--- and that includes speccing lubricants that will help preserve those tight tolerances. The car people are doing the same thing.
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