Source ?
Bro, i aint no dealer, distributor OR any wholesaler/retailer OR Lab. guy.
im just a 4x4 enthusiast, experience of 15 years only.
But Bro, i ain't no chitter chatter & internet googling around from sources / graphs / pedia's & other forums info.
& copying pasting them here 
Its' Simple: I gain experience personally, TESTED burning my time & money & then i'm able to tell my folks about it. Real & Straight cut!
I've experienced Caltex Delo Gold coking up very FAST, specially in Turbo charged diesel vehicles with Blow by PCVs'.
The REAL FACT & DEAL IS that on continuous driving over high speed upto 7-8 hours, with DELO Gold you can damage your Turbo + burn a lot of OIL, leading to engine damage at its level best. OIL looses its work & becomes like water, in simple words: Its Gone with the wind!
So finally, Delo GOLD is totally SH&***T !
uptill now, i have tested Caltex Delo Gold & Shell Helix Hx 7 only.
I compared these 2 companies only because they are easily & readily available in originality from their respective pump stations, other retailers/shops i don't trust them.
There are lots of other good OIL brands too, but don't know their original source of packaging & distribution etc.
Drive safe!
ALLAH BLESS! ( Ameen )