I use the DCK aglomerate corks that are distributed by Winexpert. I have no problem relying on them for up to 3 years, and, have had bottles for longer with no problems, however; cellaring conditions can affect your results, so your are going to have to gain some personal experience.
I have heard that #8 corks only last 1-2 years, while #9 corks last 2-7 years. That may be what they are talking about. I only use #9 corks, so I can't be sure about the #8's. #9's are harder to insert with a hand corker.