It won't effect your ability to shift, unless you decided to put it in to the transmission. It could effect your mpg due to the higher viscosity, I am not sure if the r180 can get the gl5 stuff though. Something about the GL5 or GL4 degrading the internal brass bushings.
GL4 in the manual trans.
GL5 everywhere else. In fact, the M226 calls out synthetic specifically. 75w-140. A severe gear application in the rear would work well. The front diff calls out 75W-90, and since it's used hardly as often, doesn't really need the 140 molasses.
You found it for a few cents more? What brand? Synthetic? Usually 90 runs $14-15, while 140 runs $18-20, depending on the brand.
And yeah, no ill side effects, just thicker oil to rotate through. Decrease in gas mileage is about it, but none that you would probably notice. The front diff doesn't need added protection like I said, but if its easier for you to just add the same oil front and rear, go for it. What does the C200 require? The M226 is 75W-140, but we have lockers or a standard trans, or both.
I use Amsoil everywhere but the atf. I found those prices on their preferred customer site.
75w-90 is what is recommended for the c200. But I have seen other vehicle that will recommend 75w-90 for standard duty, and a higher viscosity like 110 or 140 weight for towing and heavy duty applications.
I was thinking of just going with the 75w-110 because the amsoil site says it offers better protection without affecting mpg. The only thing is I noticed the 75w-140 is strangely cheaper than the 75w-110? So if I can get better protection, even if only slightly better, it is worth it to me.