The lime in the High-Temp recipe is added to make the mortar a bit more “fluffy” and easier to work with. Some contractors will leave the lime out all together, but I wouldn’t recommend that if you are just starting out.
The lime really does make a difference (but don’t add any more than recommend). Additionally, it prolongs the curing process of the mortar so you will have more time during the mortar application period. This can come in handy if this is your 1st, 2nd or even 10th oven build.
As @bikerbudmatt mentioned - you only need to put high-temp mortar where fire touches brick. So if a brick is exposed to flame - you will use HT mortar (but not on the hearth brick).
No matter what type of oven you build - MAKE SURE YOU PROPERLY INSULATE THE OVEN with 2 or 3 layers of ceramic fiber blanket!