I have been going to the same dealer since 1998, not very long but he is an RVer and really feels that six years is a good life expectancy for near 100 percent safe use. He has always installed all six tires at one time and given me tires with the same date of manufacture and the same warranty. If the sidewalls are good on the old tires, he gives me a discount and sends them to a retread shop to sell for use on delivery truck drive axels.
He tells me the same things every time I'm there, don't leave them sitting for long in the same spot and just use clean water on them to wash them, dressing may not hurt them but it doesn't extend their life, it just makes them shine.