Let me help you add your 8k-mile trip as a lump sum.
Before anything else, know that it isn't possible to add the 8k miles in one trip the maximum is 5k miles. However, we can divide the 8k miles and assign them to their designated dates to create a lump sum. Here's how:
From the Expenses tab, select Mileage.
Click Add trip.
Enter the Trip date of the first lump sum.
Type in the first chunk of the 8k miles in the Distance (mi) field.
Then, Start point and End point.
Select whether it's Business or Personal.
Choose a Business purpose and Vehicle. Click Round trip if it is.
Once done, tap Save. Repeat this until the 8k miles are covered.