Originally Posted By: ekpolk
Good question. But it does include a couple unstated assumptions that I think bear critical examination. First, for whatever engine is involved, where exactly does the "threshold of danger" exist? If the threshold of danger is well below the 20 wt vis range, then perhaps a 20 would be just fine. If an engine is designed so that it can't tolerate anything below a 20 wt lube, then yes, the 30 will provide a larger buffer space that the 20 can't.
But does anyone really know where these thresholds lie for any given engine?????
I don't have the slightest idea where the threshold of danger lies. My car was originally spec'd for 5W-30, now Ford has back spec'd it to 5W-20. It runs beautiful on 5W-20 and 5W-30.
I am working up the nerve to try an extended drain interval, thanks to all the fervor about them on here, and this came to mind. I don't doubt that both additive packages are comparable in terms of longevity. What I'm asking is, with all else being equal, if the underlying 30 weight synthetic fluid can withstand the rigor moreso than a 5W-20.
I see 5W-20 as a more advanced product. More difficult to manufacture, therefore requiring more technology to produce. It wouldn't seem absurd that it's lifespan as a lubricant is naturally shorter than 5W-30, merely because it is being asked to do more to begin with. Making it into a long life oil would seem to be a greater task than making 5W-30 long lived.
On the other side of it, if 5W-30 does in fact handle fuel dilution better, that alone might be reason for some to choose it over 5W-20. Particularly people for whom city driving makes up a good percentage of the running hours.
