I post a couple of vague sentences on engine longevity when using three viscosity grades heavier than recommended for no good reason, and then insults start rushing in.
Why would you elevate your oil temperature substantially by running three viscosity grades heavier? The worst thing that is happening on the seals, including valve-stem oil seals, is the oil temperature. Higher the temperature faster they will harden and fail.
My main analysis was regarding loss of horsepower and fuel economy, not even longevity issues. It's plain silly to sacrifice horsepower and fuel economy using an oil much thicker than recommended for no good reason (no towing, no high loads, etc.). It's probably also borderline paranoid and/or masochistic.
The only pro you can come up with is thicker oil film and perhaps a wider margin of protection against engine wear.
I can come up with so many cons:
Higher oil temperature
Excessive oil pressure
Reduced oil flow
Increased engine temperature
Too thick during warm-up
Loss of horsepower
Loss of MPG
Higher stress on the oil filter and substantially increased frequency of oil-filter bypass events
Thinner oil film in certain areas of rings
Increased time at startup for the lubrication system to pressurize
If the oil flow is too little, oil may not even reach certain areas
Due to the reduced oil flow, slower refreshment rate of antiwear additives
Due to the reduced oil flow, lifetime of contaminants before they get filtered out in the oil filter is longer
VVT, VVL, and similar hydraulic valve systems may not work probably if the oil is too thick
...
Just use the recommended viscosity and you will be fine. You can use slightly thinner or thicker if you like but there is usually no need for such things perhaps unless you have worn valve-stem oil seals and you need thicker oil to reduce oil metered through the valve guides. Or perhaps you want to tow or race uphill etc.
BMW 3-Series reliability. German-designed engine. Oil recommended and used: GC 0W-40 or M1 0W-40. HTHSV = 3.8 cP
Another USA driver survey. 30 worst oil-burning cars. Top offenders all use GC 0W-40 or M1 0W-40. It doesn't look like there is even one thin-oil car in the list.
