Mobil 1 oil weight

[John Larson]

Grant wrote:  "Guys - ignore those charts.
Think oil flow.
Think about the distribution fo time from cold to hot.
Read the viscosity ratings at 40 and 100 deg and extrapolate.
Those charts are obviously wrong.  Note also how every manufacturer is 
moving to thinner and thinner oils.  We're learning, slowly."

Grant

I dunno what happened to my post this morning, but I'll repeat and elaborate on the thoughts here.

It's true that most, if not all, automakers are going to the use of thinner oils.  This is an effort to obtain reduced friction in their search for improved fuel mileage.  In doing this, they've redesigned their engines with reduced clearances and smaller oil passages.  Modern engines have shrouding and insulation to stabilize temperatures, special coolants, and are made of new and exotic materials.  I believe the engine to which the original post referred (correct me if I'm wrong) is an '85 5KT, hardly a "modern" power plant.  It's a cast iron block with a plain-jane aluminum head, something that's been in common use since the mid '50s  It was designed to use a pretty early generation multi grade oil of substantial viscosity.  Mostly 20w50 in weather common in much of the US, and maybe 5w30 or 10w30 in severe weather.  Beginning in the late '80s, some Japanese cars (4 cylinder SOHC Mazdas, for sure) required 5w30, and many other manufacturers began to see this as a way to reduce friction and improve fuel economy.  It's interesting to note that the aforementioned Mazda 1600cc engines exhibited horrendous lifter clatter when 20w50 oil was used!  It would appear it couldn't get through the passages and the lifters ran dry.
Audi began specifying thinner oils sometime in the time of the C4.  Thinner being 10w30 or 10w40.  20w50 was still recommended for sustained high speed driving in hot weather.  With the advent of the 1.8T in the A4 and other VAG vehicles, 5w30 synthetic was the oil of choice, then it was changed to 5w40. Late MBs use 0w20, as do some American made cars.

My point is, cutting to the chase, you risk SIGNIFICANT engine damage running oil other than the weights recommended for specific engines of specific model years. First as an enthusiast, then a professional tech for a total of right at 47 years, I've seen oils change for a LONG time.  Running light oils in engines designed for heavier oils can, and will, result in reduced oil pressures at any given temperature.  In air cooled engines, where I started out, this translates to an inability to remove heat, effectively reducing the life of the engine.  Toasted bearings, rings, valves and seats were a direct result.  Water cooled engines, as well as turbochargers, depend on oil to both lubricate and to help get the heat away.
That's why we use oil coolers, a once rare component that has become an integral part of most engines and their temperature management infrastructure.   

Do yourselves a favor.  Consider carefully the thinking of the designers of the engine you have.  Sure, there have been steady improvements in oils, but the viscosity required when YOUR engine was designed isn't necessarily affected by what the new oils offer.  Most of us accept that running high octane fuel in an old engine made to run on low octane fuel is a waste of money, and that running lead additive is a waste of money with hardened valve seats, so why the resistance to running oil viscosity recommended by the manufacturer?  Is your personal engineering department better than theirs?  Is anything you see on the internet infinitely more trustworthy than the recommendations of the automakers?  I deal with this cr*p every day.  Bozo comes in with a part somebody said would make his car faster, louder, quieter, more fuel efficient, cooler looking, or whatever, and he doesn't want to hear it won't fit, it won't work, it'll wreck his engine, it'll make it fail the smog check. After, what do I know, other than the truth, the law, and reality?  BTW, those fuel line magnets STILL fly off the shelves at Harbor Freight!

Think about it.  Sorry for the long rant.  John