I'm confused ... because doing the math, it looks to me like the oil is still costing you more than pump diesel.

Assuming you're using US gallons (maybe you aren't),

3.78541 L / gallon

Diesel $1.45 / L

1 gallon diesel = $5.49


Oil:

20 liters (5.28344 gal) for $30

Or $5.68 per gallon