My point about the transmission is that they’re mass produced and machined on a jig. The mainshaft assembly drops into place and the bearings are located by dowel pins. There is of course slop lengthwise in the shaft. Subaru offers about 10 different 3/4 shift forks (same on 1/2) that are numbered.
The ends of the fork where it fits in the synchronizer sleeve is machined with an offset. There are about 5 forks machined one direction and 5 forks going the other. The measurements must be taken on the synchronzier assembly and gear sets and the sleeve centered. This means the sleeve is held in the proper place by the fork and that is done by selecting the proper fork.
If the machining was dead on there would be no need for 10 different shift forks and going through the procedure of picking the right one. (I would add that this is not a matter of .0002 or something like that. We’re talking .020, .030, and so on.
As I said, I do understand your point and agree about the machining accuracy but do not agree that it makes any difference on engine longevity. Example, and say the oil clearance is given as .001 - .0024.
Assume one engine has journals all at .002 and another has a tighter fit with all journals at .0016. Does that mean the latter will be a better motor because of that .0004? No.
Regarding heavyweight oil I don’t necessarily consider it the kiss of death on an engine. When my cars reach high mileage I usually run 20/50 in the summer and have done this for decades with no problem. My old Mercury had a diet of 20/50 for about 250k miles after it reached the 150k mark.
At some point there is enough wear on the bearings and journals that a heavy oil can help keep those bearings seated on the rods. With thin oil and enough wear it’s not unheard of for a rod bearing to swap sides and cause a catastrophic engine failure.
Would I use 20/50 in a low miles motor? No, but there can be a place for it later on.
Same goes for my motorcycles although that veers off into the air cooled thing. My HDs run the factory recommended straight 60 weight although I do not use the overpriced HD branded oil.
I run 60 weight Aero Shell, which is an oil used in aircraft engines.
Again, I apologize if I come across as combative. It’s not meant that way at all, 