has been using the Castrol 10w/60 with hydaulic tappets and has been very happy.
Let me see what my friends alfa 33 factory specs say.
He did state that his sticker says 10w/60.
I would have no issue with 15w/50 in the UK with your climate but here is a different story in summer.
The engine runs cooler, quieter and faster with the 60 viscosity oil.
If the tappets were having issues you would hear them for sure.
Mine were bleeding out when getting too hot before.
Also the clearance on the main bearings can range from 0.028mm-0.063mm. Mine are between 0.055 and 0.060mm more or less.
A difference in 0.020mm can make a big difference in oil pressure, they will bleed too fast if the oil gets hot and thin resulting in the bearings getting too hot.
Fact specs state 15w/50 but for brand new crank journals with tight clearances.
IMO sometimes you need to boost the oil pressure to keep the bearings cooler.
If you have a nice stable temperature then the flow rate is good. Too hot and something is wrong.
I'm switched from 10W-40 (Selenia) to 10W-60 oil. The 10W-60 brand first was Selenia (Racing),
now i'm using Castrol Edge Sport. No extra oil leaks detected since then.
The oil pressure is considerably higher at hot engine at idle in heavy traffic compared to the
10W-40 viscosity grade oil. (I have an aftermarket oil pressure gauge installed.)
How's the 10w/60 going? You have hydraulic tappets also don't you?
is the TWS the renamed Edge Castrol oil?
BMW spec Castrol TWS, which is Castrol Edge (not "Edge Sport") in most markets now, but is still sold as "TWS" through BMW.