Originally Posted by StevieC
If an OE spec's a 20wt oil and then has to back peddle it to a 30wt because of some issue that it can't handle the 20wt without some sort of problem. This is poor engineering on the OE's part to have built the engine for the oil specification they printed in the manual. Whether that be enhanced oil cooling, changing bearing design or whatever it is that is stopping the engine from running on the 20wt like any other engine that is spec'ed for it and runs fine on it.Hope that makes is more clear.
The only case I can think of off the top of my head where something spec'd for a 5w-20 was changed to a 5w-30 was the EcoBoost IIRC. And that was, at least in part, due to fuel dilution, which is an issue with DI.
What Shannow has mentioned is that outside of CAFE markets, you'll find that *insert your favourite engine here* often specs a heavier lubricant. He's provided a couple of examples with engines spec'ing Dexos 2 instead of Dexos 1 for example. Why? I assume both are equally as available.
So it isn't that the engine is necessarily designed to be run on the 20 but rather that it will achieve an acceptable lifespan under typical operating conditions running on that grade, so it is a non-issue. That doesn't however mean that the 30 isn't a step-up in performance, even if it isn't significant for the typical engine under a typical usage profile.
Australia is hot, so usage in Australia would be more akin to operating in Texas or Arizona. However, CAFE is nation-wide, so the same oil that has to work in Minnesota also needs to in Texas. That's a compromise. Ergo, mechanisms are put in place to ensure that any issues will be reasonably mitigated, be it via specific output throttling or otherwise. That doesn't make it any more ideal, it's simply accommodation of limitation.
As I said earlier, it comes back to chasing the delta between startup and operating temperature. The smaller you can make this, the more fuel you save, hence the pursuit. Because of the limits being pushed more Moly is being used to counteract the increased operation in boundary at that end of the Streibech Curve. On top of that, as already mentioned, controls are put in place to control oil temperature and thermal watchdogs used to limit output if thresholds are passed. All this in pursuit of that fraction of a MPG.
Back in the day of course you had a table (like the one posted above) that showed expected operating conditions and the respective recommendations for those conditions. As oils have improved and grades like 5w-50, 0w-40...etc have come about, that seasonal guide is less important. The message of that guide however should not be dismissed. There is no single perfect grade of oil. When it's -30C and your oil temp can't get up to operating, even a 5w-20 may be too heavy. When it's +30C and you are hard on the pedal heading up a mountain pass, that 5w-20 may be too thin to be "optimal". That doesn't mean catastrophic failure, simply that something heavier would do a better job. Perhaps there are parts now in boundary that would be better in hydrodynamic for example.
What these safeties don't account for however, is fuel dilution, which reduces viscosity as well. So while you can manipulate sump size, cooler choice and thermal-driven output limiters, if you are not accounting for additional viscosity reduction via fuel dilution, you increase risk of damage as the limits of those parameters are approached. This will become more of an issue as thinner and thinner oils are introduced. It's one thing when your 0w-40 ends up a 0w-30, it's quite another when your 0w-20 becomes a 0w-12. The safety margin gets smaller and smaller and operation in boundary increases and so we rely on additives to deal with that. That means no "pile of failed engines" but it also means that we are stepping out of the range of optimal wear protection for the sake of pursuing fuel economy.
