Originally Posted By: DemoFly
For the NP231, NP242 and NP249 I think the ATF spec isn't the best.
Sure, it's great for lubricating the chain and adequate for the planetary gears.
But it isn't sufficient (IMO) for the input and output shaft bearings. They are roller and ball bearing.
So,
if the idea is to use an oil that provides the longest use without maintenance or repair, I use synthetic 5w-20. I did a non-scientific test submerging a 242 rear output shaft bearing in ATF+4 and 5w-20 and the bearing has audible metal to metal contact with ATF+4. It's silent with 5w-20.
After changing the rear output shaft bearing twice in 20K miles, I was led to try something different.
+1
Concur with you that roller bearings and ,if I may add gears, require higher MOFT in 5W20 for components wear protection.
Heck I would suggest OP's 15W40 offers even higher MOFT than 5W20, thus is doable for him in this Xcase application.
MOFT thicker/higher than 'basic' film thickness offers stronger wear life protection, but whether that's necessary would be another thread for debate as to its merits when issues of FE is enthusiastically promoted.
In oil viscosity grade selection, I'm biased towards your concept of "
if the idea is to use an oil that provides the longest use without maintenance or repair" which , I believe, led our forefathers into developing Lubrication as an industry as we see it today .
With the multiple trillions of resources being incurred in the industry,I believe, it's unfortunate that viscosity-related components failures is still rampant, which could be 'eliminated' or reduced to insignificance by maintaninig adequate 'margin of safety' in MOFT.
I consider your non-scientific test as offering another qualitative evidence on high MOFT vs low MOFT.
Btw,DemoFly we are but minority here.
