I think your concern is that the Castrol R may have oxidized to a sludge or even solid over time in spots? Known to do that. I have no personal experience w/that, but have read of it on the SOHC4.net site.
I've used, w/great success, ATF as a combination solvent/lubricant in engines with sticking rings, gunk, etc. I'd be tempted to use a cheap motor oil in the right grade for the engine and ATF in 50/50 mix, run it at idle and low throttle w/o load for 15 minutes. Drain, repeat as indicated.
This is a very sold-school engine cleaning technique with many decades of proven results. Were it my bike (I'm presuming this is a bike of course), I'd have no qualms about doing this. 'Cept on a Harley with their very different oiling needs. I'd still do it there, but a different ratio.