I would have to say that the inline 6 last so long for a few reasons, for one I would have to agree with the side friction difference, it doesnt seem like much but when you think about the engine turning an average of 2000 RPMs constantly at highway speed that friction adds up. Aother reason would be the 7 main bearings and one that wasnt mentioned would be that the inline 6 is one of the only engines that is actually perfectely balanced, what I mean by that is that there is no dry spot in piston stroke, with a 4 cycliner there is more of a pause between strokes and piston detonation because of its design, a V design engine is thowing pistons at a different angle and there is always more resistence because of this explaining the lack of torque. The inline six always has pistons at top during the firing order and hardly has any resistence, the low revving probably contributes to the notorious 4.9 engine life. Ford did good on this one.

 

The main reason the I6 lasts so long is its low rpm operating environment, and it has good oil flow capacity to all major components.
Having 7 main bearings or thicker castings doesn't do a whole lot in my opinon. Correct application of the right materials are far more critical.
I am going to have to disagree here.
Even if the piston weighed 100lbs, the side loading force due to gravity is miniscule compared to the forces due to combustion.

A V shape or boxer, or any other shape where the pistons aren't vertical has no reflection on cylinder wall wear - and if it did you have much bigger issues at hand.

 

a boxer engine is a totally different ball game, your talking different weight, different counterbalances and the whole works, not to mention usually higher RPMs and only 4 cyclinders, granted its a good motor but it definitely doesnt run as smooth as a I6. A V8 is constantly fighting itself and working its self to pieces, the originas V8 was one of the last to be invented because of the difficulty in keeping the engine in full rotation (this being where to counterweights or balances came into play later added to the inline engines to make them run smoother) the V8 is poorly designed for the most part and only good for about 250,000 at best, the 6 will run for 500,000 with little or no problems if taken care of. Oh, and the side friction would come in on the exhaust stroke when there is no compression on the piston, its just a dry and hot strok the one of the sidewalls.

 

I agree with crocadile dundee lol just kiddin big350! Even if the pistons are at an angle, thats what all this high-tech oil is for...to reduce as much friction as possible. If the theory of the piston wearing stuff away in the cylinder (which would mean rings being fryed) then an averadge V8 with high miles would burn oil, because the friction created would wear out the rings. I've seen (and many have) V8 gassers with 100's of thousands of miles on them, and dont burn oil excessivley. If the theory was true, then they all would. Hell, my I6 has 167k on it, and it burns 1 quart between 3-4k oil changes. my buddies z71 has 217k on it, and doesnt burn anything noticeable.

 

Good be valve seals on the inline I6 300, I guess we all have our opinions, I am just talking from experience. Doesnt really matter that much to me.