Grade and pack size are the item
A 20W-40 and a 15W-40 are different products and so are the one-litre and the twenty-litre packs of each. Entering them generically makes stock useless and rate errors routine, and supplying the wrong grade to a garage is a complaint you will hear about. Grade, brand and pack size on the item mean the bill states exactly what was supplied.
Litres and packs on one bill
A garage buying a barrel and a few one-litre packs in the same visit expects one bill with correct arithmetic. Holding the unit and conversion against the item lets you sell by pack or by litre from the same stock, and the closing figure stays true. Rate per litre differs across pack sizes, which is exactly why pack size belongs on the item.
Garage and workshop accounts
Your regular buyer draws oil through the month against jobs he bills at the end. Small amounts, frequent visits, a running account. Because the ledger builds itself from the billing there is no second entry, and a monthly statement settles the account while the work is fresh. A credit limit stops a garage that has slowed down from continuing to draw.
Company schemes and targets
Oil companies run volume schemes and targets, and hitting them depends on knowing where you stand before the period closes rather than after. Sales by item and by brand over the current period answer that in seconds, which is enough time to push a line that is close to a slab. Shops working from memory find out when the credit note does not arrive.