Like Pennsylvania crude oils that were considered the "Best" available for automotive oils, some auto myths refuse to die.
If the oil seals and the shaft they run on are in good condition, they will not and should not leak.
As for Castro oils, I loss all respect for them when I learned they were selling Synetec as a synthetic oil when it was not a true PAO based oil. Mobil sued them and proved Castro was more marketing than substance.
Amsoil, another early synthetic oil supplier gets their PAO base from Mobil.. or better to simply use Mobil One, get it from the source.
IMO, there is no reason to use anything less than the best of most modern lubrication technologies available.
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As for what fluids goes into the cars these days..
*Mobile One W5-40 turbo diesel truck oil for all engines in the fleet. Changed twice a year regardless of mileage. Got this one from the Eye Sore Racing folks after a long discussion about oils with one of their engineers.. who works for Garrett.
*K & N oil filter each oil change. The HP2004 (FORD- V8) fits the x1/9 and SAAB. As for Fram filters, they have a tendency to self-dissolve into the engine... not good for the engine at all.
*Redline MTL for the transaxles..
*G05 coolant (30% G05, 70% distilled water, no snow in this part of CA) with Redline Water Wetter. Changed every 2-3 years or so.
*ATE super (DOT4+) blue or red fr brakes and clutch.
*Redline CV-2 or Neo Oil, CV500 for the CV joints.
I've always used the cheap stuff in the past (never owned a sports car before now) so I don't know too much about the high quality oils but isn't there some risk involved in switching from natural to synthetic? I've heard you are more likely to leak.