I always used 20/50 in my xflow Sylva Striker and use the same in my 2.0 Pinto Robin Hood Exmo

10w/40 fully or semi synthetic is fine for x-flows. You have to remember that when they were first produced just about the only oil available was 20w/50 mineral and it was rubish compared to todays oils, even the supermarket brands dare I say. Viscosity does not tell you anything about the quality of the oil, you need to find the API ref on it, should be SJ, SL or SM, avoid anything lower.