From this thread on the full synthetic Castrol 10W-60
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2988455
A bit of history from Doug Hillary in a 2013 post.
Originally Posted By: Doug Hillary
Hi,
Yes as OVERKILL has alluded too I was involved with this product very early in its life - late 1960s. It was really a road going version of the famous Shell R30 and R40 easter based racing lubricants. My family used these in racing motorcycle. I was involved in my Career training with Castrol via BMC
The road going version was Castrol Formula R Synthetic - rated API SE. It had a 15w-50 viscosity, was red in colour and of course emitted the beautiful castor based odour in use
In the 1970s and 1980s I used/field tested the probuct in many diesel and petrol engines as it evolved and this involved hundreds of UOAs. I visited Japan, US and Europe to deal with various engine manufacturers and in the use of this product. In some small high speed diesel engines (Kubota,Isuzu etc) we ran up to 3000hrs without change
Two versions were later marketed - one "exclusively by BMW". I met with some BMW and Castrol engine/product development people at the Nurburgring a few years ago at a 24Hr event. They spoke so highly of the product it still amazes me. They also told me that the last of the German Castrol development team had recently died. They said that I was probably the last of that mob - wow............!
A great product in the correct application
OP, I'm sure both Castrol & Shell (Pennzoil) make a great product and I would happily use either. I have used lots of Castrol 10W-60 over the years, and right now I am using GTL Shell Helix Ultra 5W-40 (the other Ferrari grade oil). I would use both with confidence and would consider them equal ( without hard data to say otherwise).