Oils cause a bit of controversy over how much and what type/blend. Some engines need more than others and often it comes down to the personal experience of many fliers with a particular engine. You might have noticed from the OS instructions that they say synthetics are "permissible" but give a caution. Just under that there's a table for fuels A and B and that shows 20% castor. This may be specific to the LA series of engines because of the plain bearing crankshaft which really needs castor for long life. Conrods love castor too.

With oils it's near impossible to have too much but you can certainly have too little so I prefer to err on the safe side and never use less than 20%, that being all castor because I don't fully trust synthetics (a personal opinion). With an LA and a synthetic based fuel I'd add castor to get 22% total oil to be on the safe side. Don't use medicinal castor though, get something like Sig castor because it's cheaper and won't have the possibility of sugar added to it.