I want to use the best filters for dust and allergens, but don't want to exceed the capacity of the furnace fan. What is the highest filter rating for this furnace? I checked the manual but could find no mention of that.
Thanks!

Read this.
https://www.safehomefilters.com/index.php/common-questions/merv-vs-resistance.html
There is no furnace rating. Keep an eye on the filter. If it clogs up too quickly, drop to a lower MERV.