Amazon has Rotella T6, 1 Gallon 3 Pack for $57.70 after $10 digital coupon. Shell is also offering an MIR of $5 per gallon (up to 4 per household) . Net price per gallon after rebate is $14.23.
You can also buy a single gallon of T6 for $16.36 after $5 MIR which isn’t a bad deal either.