Jake - You can use whatever oil you wish...and I've got a feeling that you are going to do so regardless of what I say. But, for the benefit of others that may read this thread, there are a few points to make:
1) If you notice, Toyota also recommends a 10W-30 for the same temp range in the page from the manual. In fact, the 10W-30 covers a wider range on the low end of the chart. This manual is pushing 20 years old...0W-30, 5W-30, and 5W-40 oils were unheard of back when this was published. Oil formulation has come an awful long way since. Using your logic and this chart as sole basis, it would be just as appropriate to advise Cory to use a 10W-30. Since a 5W-30 API SM grade oil will out perform the oils listed in the original manual hands down (API SF...it's now obsolete), use of a 5W-30 oil is even better. Use of the owners manual chart to determine the oil you use is valid, but it is also outdated...to base your contention on it completely ignores the modern API SM oils that are available today.
2) You are correct about the oil flow to the cooler...above 2500 rpm or so on a healthy oil system, oil is flowing to the cooler regardless of viscosity. This is to dissipate the addition heat transferred to the oil from the bearings, etc. Are you trying to tell me that an oil that flows more freely/quickly in this regime is not going to help rid the oil of heat by flowing through the cooler faster?
3) A 20W-50 is the problem...so is a 15W-50...both are way too thick at start-up. Try to find a 0W-50 oil...good luck. The closest I can find (commonly available) is Castro SynTec 5W-50...wonder why that is? To get the oil to meet SAE viscosity and API grade requirements, viscosity modifiers have to be added to the oil formula. To be able to meet the wide range in a 5W-50 oil, a higher percentage of modifiers to base stock have to be used...that results in less actual base stock oil in the bottle volume wise. In addition, the oil is more subject to "viscosity creep" as it goes through the heat cycles of the motor...what that means is the oil will turn into a 10W-50, 15W-50, etc over time. The viscosity range will decrease...that, plus the difficulty of meeting SAE/API requirements is why you do not see a 0W-50 oil.
4) First I want to say that Dr Haas knows more about motor oil than anyone else I know...he is a true authority and has proved his hypothesis on some very high priced cars....ones that recommend a much heavier oil for use in the engine. It's pretty easy to take a quote from ANY document to justify a position...the overriding theme Dr Haas was trying to get across is in almost EVERY case a thinner oil is better. In fact, what I'm trying to get across is in Motor Oil 109 and 201...the difference in flow between a thin vs. thick oil.
You took "absolute viscosity" out of context...it was a generalization. I am going to be more specific...take a look at this oil data sheet on Castrol SynTec (a Group III hydrocracked base stock...not a true syn oil):
Note the viscosity at 100 deg C (ops temp) for:
0W-30 = 12.1 cst
5W-50 = 17.4 cst
That is a 43.8% increase in viscosity...over 4 times more than the 9.1 percent you quoted. Also note the 5W-30 has a viscosity of 9.7 cst at 100 deg C...it's actually thinner than the 0W-30 oil and would produce a 79.4% difference in viscosity. Unfortunately, Castrol did not publish viscosity numbers for 40 deg C...so, using these oils to make a "cold" comparison is impossible. However, Red Line (a Group V ester base stock...a true syn oil) publishes figures that will make the point:
Note the viscosity at 40 deg C for:
5W-30 = 62 cst
5W-40 = 94 cst
That is a 51.6% increase in viscosity for the 5W-40 at 40 deg C. Keep in mind that the colder oil is, the more it thickens up...the hotter it is, the more it thins. This is true for every grade and SAE viscosity oil out there. Imagine what the viscosity would be like at 20 deg C...or 0 deg C. And keep in mind a 5W-50 would be even thicker cold. Let's compare the oil weight you recommended... a 20W-50 vs. a 5W-30 both hot and cold (again, using Red Line):
Viscosity at 100 deg C (ops temp) for:
5W-30 = 10.6 cst
20W-50 = 19.8 cst
This is a 86.8% increase in viscosity.
Viscosity at 40 deg C for:
5W-30 = 62 cst
20W-50 = 148 cst
This is a 138.7% increase in viscosity. This is EXACTY why I said putting a 20W-50 in your motor is not doing it any favors...these are the numbers and they do not lie.
Considering the above discussion (based on fact...not opinion) I am very sure that an oil with viscosity characteristics of a 20W-50 would take longer to travel down the entire length of the crankshaft to the #6 bearing. The oil will take longer to get to the top of the motor and travel down the cam shaft to the journals and valves. The volume of the spray pattern produced by the oil squinters will be affected...all of this is simple fluid physics. Plus, engines have been designed to use an oil at ~10 cst for the bearings for a long time...only in the last few years have the designs moved toward the 0W-20 oils.
5) The oil pump relief valve is also set at 40 psi...that's why shimming the valve will produce higher pressure in the motor. This is the reason you don't see pressures over 40 psi on a stock pump...ever consider the higher pressures caused by the resistance of a 50W oil (both hot and cold) is causing the relief valve to function? The oil is being diverted right back to the pan in this case...you've effectively cut the volume of oil going to the parts of the motor that need it. A case in point where pressure does not equal flow.
Like I said Jake, experience combined with knowledge is very powerful. But, the only real piece of data you were able to point to was the Supra Owners Manual...the one that also shows a 10W-30 over a wider temp range and an obsolete API service grade. Perhaps you should start looking for an API SF grade 20W-50 to meet your personal requirements.
The contention that a 20W-50 or 15W-50 is "better" is a myth...one that is so engrained in many mechanics minds and passed along for far too long. However, JJ is right...it's like shoveling sand against the surf...opinions are something hard to change. Sorry, but it just does not add up....every person I have convinced to dump the 20W-50 has been happy with the result. My motor runs smooth, especially at high rpm...I'm running a 0W-30 syn oil that gives me 45-50 psi over 2500 rpm (shimmed pump)...in Phoenix and have done so during the summer at 115 deg F outside. Plus, I enjoy a little better gas mileage
I'm sorry you've chose to ignore the science and stick with your gut feelings...I wish you (and your motor) well.
