OncInc

I have 2 guesses
1. Like Cwong said.  The McD Fries are thinner in size.  When they deep fry them, the water is all gone.
2. When the oil is heated too long,  regular oil becomes trans fat which can be used in the food industry as a preservative to extend self life.
BTW, there is a trans fat ban in Canada (not sure if it is mandatory or not).  Perhaps,  McD is still not complying.

CWong

嗰日到見到麦記个员工喺炉底嗰桶万年油借油，補番滿炸炉內嗰啲咋緊薯條嘅油！
有時麦記啲薯條或者薯餅食落苦苦哋o架，你就知啲油有几新鮮了！食到生Ca都得！

RayRay

Should throw in some Coke with secret formula to "disolve" the fries   Try another experiment la.