The Best Paper for Long Term Storage
I don't worry too much about paper anymore. At one time, and this was in the 1980's, it was difficult to find acid free or alkaline paper. Our libraries were burning up, and from within, because most of the paper the books were printed on was highly acidic. Paper became brittle quickly.
Now, it's different. The paper mills have gone to an alkaline process. But they did not want to change.
If there were one person to thank, it would be Ellen McCready, who with tireless dedication edited the Abbey Newsletter for many years and then, the Alkaline Paper Advocate. She showed how money could be saved by the mills, and finally, after many months, they had no arguments against change.
So nowadays, the bond paper you get at the local office stores, even the cheap stuff, is mostly alkaline. That's a good thing, but now you ask, "Why then do I need to consider the finer papers?" Why even have the 25% cotton (and sometimes the 100% cotton) papers? And, what about the Microchamber papers (active against impurities) and the Permalife papers (buffered with additional alkalinity)?
You can find a good basic discussion of papers by reading the Conserv-O-Gram: Judging Permanence For Reformatting Projects: Paper & Inks. The short answer to the question about papers begins with the fact that over time, the alkalinity goes away. Even alkaline papers will become acidic eventually. So the real reason you want papers with 25% cotton is because, over time, their fibers are longer, and the alkalinity that is in them will last longer. You should use the best paper you can for your time capsule and long term legal and genealogical use.
We always carry Permalife paper because they have 25% cotton and have been tested to have a 300 year lifetime (assuming proper storage). That paper is also buffered so that its alkaline content is higher than normal bond paper. It has a weight basis of 20 lb. and it carries a watermark.
It makes sense that the basis weight of the paper will hold up longer to loss of alkalinity. So if you have a choice among paper with weights of 20 lb, 24 lb, and 28 lb, the 28 lb will last the longest - assuming all the other things are equal (25% cotton, for instance). But throw in the addition of buffering and, in the case of Microchamber and Alpha Rag (which have active filters to remove impurities) will change the game a bit.
The cheapie sulfite bond paper you can get at your local office store is fine for everyday use. We use it here and print our pamphlets on it. But the fibers are short, and although the paper is alkaline, you need to know that it won't long be 'acid free'.
So, we do recommend this Permalife paper for time capsules. It is made in the USA, is 25% cotton in 20 lb weight (available in letter size 8.5 x 11 inches and legal size 8.5 x 14 inches). It has been rated with testing to be good for 300 years in storage. We include the letter size in all of our preservation kits. The letter sized Permalife is often specified for Masters and Doctoral Theses. And now the legal Permalife, being 25% cotton and ivory in color, is now considered acceptable for NSDAR applications and for many legal papers.
We also sell a fine letter sized Microchamber paper. It is 25% cotton, ivory in color, and has a basis weight of 24 lb. It has the zeolite filters and other active filters that help guard against impurities. This paper works great when you are storing really bad papers such as old newsprint and pulp fiction (old paperback books). These papers are best for those conditions that call for big protection. We don't feel that most current time capsules have items inside that are highly acidic (except for newspapers), so we don't include the Microchamber bond in most of our kits (except for 11002). 
For what we feel is the finest for DAR and other legal and lineage applications, for paper that will withstand the test of time and heavy usage, we have the legal sized 28 lb paper that is 25% cotton and ivory in color. It's heavy paper with long fibers, and is long lasting (but beware - its surface is smooth, so take care with handwritten signatures to make sure they are dry). The paper would make a nice paper for City Proclamations and any works on paper that call for a substantial weight and longevity.
Look for 25% cotton content, and look for a basis weight that is at least 20 lb. and appropriate for the use you intend for it.