Originally Posted By: DeepFriar
I confess to being scientifically dumber than a post in this area and basically grew up in the thicker is better school (true of the dinosaurs that were around then too). I used a 0W40 with a VI of 190 (just looked it up and wondered "how" they might accomplish this). It appears to contribute to a NOACK of 9 where others in the lineup look to average about 6. Can someone knowledgeable comment on when is too much, too much? I apologize in advance if this is the wrong place to ask.
Will add the source data:
http://www.redlineoil.com/content/files/tech/Motor Oil PDS 5-13.pdf
There are mostly just 2 things to consider: The HTHS at a hot 150 degC, and how well the oil flows when "cold". An engine running at normal temperature only sees the HTHS. An engine starting up in cold weather sees a bunch of thick honey.
Therefore, I believe you can never have too much VI, due to the fact that you want an oil that is reasonably not too thick on cold starts, while nailing the HTHS target hot.
People disagree here on what's "reasonable". Every oil is too thick at, say, 20 degF and under. Current VM technology can't make the oil be around ~100 cSt where I would like it to be when at 20 degF, its always too thick when the mercury drops below freezing.
The other problem is that the more VM chemicals you blend in, the more risk you have of destroying the viscosity curves set up when the oil was new, since this stuff tends to break up with a lot of heat and pressure. Chemical problems. Some worse than others, but when you use Redline, I presume the POE basestock is very stable over time not much VM is needed there, so you should be good to go longer.
As far as NOACK, the lower the better. Less than about 10% is considered plenty good.