Guava jam is my all-time favorite and I like to use a thick layer of it for maximum impact. When I have the option, I like to use half Hawaiian Guava (Hawaiian Sun brand usually) and half of a Latin style - the variety that is so thick you can slice it. I put both of those in a food processor and whiz them together until smooth.
However, any type of jam can be used in this recipe. Tart jams tend to be the most well-liked. Blackberry is always a fan favorite but Raspberry is equally good in my opinion. I use seedless whenever possible - Knotts Berry Farm is a good choice. When using Seedless Jam, one 17.5 oz jar should be all you need, volume-wise.
3 cups AP Flour
2 1/2 cups Quick Oats
2/3 cups Sugar
2/3 cups Brown Sugar
1/2 tsp Baking Soda
1/2 tsp Salt
12 oz Butter
2 cups Guava Jam
Stir dry ingredients together. With mixer on low, add butter in pieces & continue to beat until mixture is well-blended and resembles wet sand, about 2 minutes. Press 2/3 to 1 cup of mixture into bottom of 9x13 baking pan. Bake at 350 degrees until light brown, about 15-20 minutes.
Gently spread jam preserves over hot crust. Sprinkle remainder of crumbs over Jam layer. Bake at 350 degrees 25-30 minutes until golden brown.