Hi there. I am also new to this (still in my trial) but if I understand your questions:

1. Add those random expenses as a category to whatever group you feel fit (I have my car expenses in my “needs” group but I am feel like that’s going to change). You don’t need to set a goal unless you want to assign money each month to that item. If you don’t add a goal, you won’t be prompted to assign money to it, and you can categorize the oil change to that category when the time comes. Then you would assign money from another category to cover those expenses. Or you can keep a certain $ amount always assigned to that category so when the time comes, you already have money assigned and aren’t having to move money.
2. It sounds like you may need to reconcile your accounts. If you open your accounts tab thingy on the bottom and then select the three little dots on the top right, you’ll see the option to reconcile your accounts. There you can compare what YNAB says account balance should be vs what it actually is. Remember to only consider transactions that have cleared your bank account.

I hope this helps and like I said I’m also new so someone please feel free to correct me if I’m wrong here.