Tires: Acura MDX 2008 AWD tires in NYC, awd vehicles, acura mdx

Question
I have 45,000 miles on my SUV. I had Bridgestone P255/55R18 104H M+6 tires on my car from the dealer. Treadwear 400, Traction B, Temp B. I was told by some that I need the same tires because if I don't get the same ones, I would ruin my transmission. A transmission guy said if the tires are the same size, they are fine. He priced Nankang 255/55R18 109V M+5 tires. Much less expensive. Are these tires ok and is it true what is said about the transmission? Obviously the replacement tires would be the same size. Also, I was told that some tires have directional arrows presumably for how they direct the water away as you drive. Can't put them on the other way or they don't work correctly. Would this be a problem in that you can only rotate the tires front to back and not side to side? Are there other issues I have not identified? Would this void my warranty? I suppose I could use some basic education on this topic. The Nankang have a good rating from what I can tell.

Answer
Debra,

First, many AWD vehicles are sensitive to differences in tire diameter - so the standard recommendation is to replace tires on these vehicles in sets of 4 and maintain a regular rotation schedule so the treadwear is even among the 4 tires.  Failure to do so may result in drivetrain problems - which is probably what was being referred to.

Directional tires?  They aren't a problem, but I'm not a big fan simply because of the rotation issue.  No, it doesn't cause problems, but it is an un-necessary complication.
 
Void the warranty?  I assume you mean the vehicle warranty - and no, so long as you replace the tires in sets of 4 and maintain a regular rotation practice, the warranty will not be voided.