I had a couple of significant roles on Marathon's Garyville Expansion project last decade. We essentially built a grass roots 250,000 barrel per day conventional petroleum refinery with hydrocracker & coker for medium to heavy crudes in parallel tied in with the existing 275,000 BPD refinery with FCCU & coker, for $4 billion. No lubes production, the hydrocracker unconverted oil is fed to the FCCU where it readily cracks to fuels and petrochemicals, but say we even throw in another whole $1 billion for a state of the art lubes facility, that would be $5 billion.
Shell's Pearl GTL facility produces about 120,000 BPD of products including around 25,000 BPD of lube base stock for about $25 billion capital cost thus far - half the production volume for 5 times the cost. Even with free natural gas both for feedstock and utilities to eliminate big chunks of variable operating cost, the difference of an incremental $20 billion for half the production volume can't be ignored and just buried in other corporate line items. That's equal to BP's Gulf of Mexico incident settlement!
https://www.google.com/amp/s/www.forbes....-3-billion/amp/
And yes what we call FCCU Light Cycle Oil on this side of the pond will already have its cetane upgraded as part of the hydroprocessing to get the sulfur levels down to Western ULSD specs so the value of the 70 cetane GTL distillate is limited as SOJ notes. You realoy need an ultra low sulfur but otherwise poor quality = low cost / value distillate source to nlend with to maximize GTL diesel value.
SOJ, gas oil over here refers to FCCU & some hydrocrackers feed not products, a generic catch-all for material too light to be asphalt but too heavy to be diesel. We have atmospheric gas oil, light vacuum gas oil, heavy vacuum gas oil, some configurations also produce a medium vacuum gas oil, and coker gas oil. Extraction units produce deasphalted oil.
FCCU products are typically called FCC LPG, FCC Gasoline which can have seperate Light and Heavy draws depending on configuration, FCC Light Cycle Oil, some folks pull a FCCU Heavy Cycle Oil but most just use this as a pumparound, and FCCU Slurry Oil - with of course some regional differences in terms here!
Nothing like lack of common language to foster greater Anglo-American understanding and communication!
If I had to pay that kind of money for daily driver oil, I'd change it maybe every 5 years ... By-pass filtration would be a must 
