I personally think that glue-ins -- short quotations, often from prophets and apostles -- are a wonderful teaching and memory tool, and I highly recommend using them. Reading a glue-in (even several years later) can often help invoke the same feelings you or the students felt when you first read it and glued it into your scriptures. 

I've often been asked: "Do glue-ins stress book bindings?" My answer is always: "Yes -- but it's definitely worth it!" 

Here are a few ideas to enable larger numbers of glue-ins to be added to scriptures (regardless of the paper you choose to use): 

• Place glue-ins slightly to the left (on left hand pages)  or to the right (on right hand pages) of the inside binding  of the scripture page -- instead of forcing it "into the  binding" as far as possible. 

• Whenever possible, glue your glue-ins vertically (not  overlapping) on the same page (so that several glue-ins  can be included without changing the thickness between  those pages). 

• Glue some glue-ins at the outside left margin (on left  pages -- gluing down the left side of the quotation)  or reserved for right hand pages. (This makes your  scriptures "sit fatter", but it helps relieve stress  on the binding.) 

• If multiple glue-ins are going to be placed between the  same scripture pages, you may wish to consider creating  a back-to-back quotation (with glue-in quotations  on both sides of the glue-in page). (This is usually  only time-effective, if you have 4 or more glue-ins to  be placed on the same page in the scriptures.) 

• ...and, I'm sure there are many other ideas that will work, too. 

If you decide you would like to use thin paper for glue-ins, there are many kinds of paper that will work nicely. 

Lightweight (9lb) onion skin works fine on most printers -- but it can be fickle to photocopy. Onion skin is available through many office supply and craft stores. 

If possible, though, I recommend that you visit a paper supply store in your area so you can see and test any paper you are considering using (rather than ordering it sight unseen). 

After you find the paper you're happy with, I recommend that you take it and the glue-ins you wish to photocopy to a copy center (in a nearby office supply store or printer) and have them make the copies for you (instead of trying to copy them yourself on the church copier). 

Many copy centers that I've worked with will reimburse you if they damage any of your thin paper during their photocopying. (Make sure you ask them about their replacement policy before you begin.) 

I have personally found that regular, white paper (20 lbs. or less) works fine for most Seminary lesson glue-ins, but there are certainly reasonable differences of opinion on this topic.

I strongly recommend, though, against photocopying glue-ins onto cardstock or heavy specialty papers and then gluing them into scriptures. Glue-ins copied onto cardstock can make excellent bookmarks, however, especially if they're laminated or covered with clear, wide packing tape (which can be used as a low-cost alternative to lamination).

In my experience, the difference in paper thickness of thin pieces of paper only becomes important (to help preserve the binding in the scriptures) if you or your students are adding a very large number of glue-ins to your scriptures. (I have a large number in my personal scriptures, but I use regular, white 20 lbs. paper and just plan on my scriptures wearing out every decade. I use regular paper for my personal glue-ins so I can write notes, information, and impressions on the backs of my glue-ins. In addition to the quotations on the front, the backs of my glue-ins become like little note pads strategically placed throughout my scriptures.)