If you are changing it every 8-10k kilometers you can use whatever is cheapest that meets the manufacturers specification. The "100% synthetic oil' you have been using is a good oil and could coceivably deliver much longer change intervals but you will need to monitor/test it.
Selenia WR 5W40:
40.0-50.0 % Distillates (petroleum),
hydrotreated heavy
paraffinic (649-467-00-8)
CAS:64742-54-7
EC:265-157-1
Asp. Tox. 1, H304,
DECLL(*)
01-2119484627-25-
XXXX
1.0-2.0 % Zinc
bis[O-(6-methylheptyl)]
bis[O-(sec-butyl)]
bis(dithiophosphate)
CAS:93819-94-4
EC:298-577-9
Skin Irrit. 2, H315; Eye
Dam. 1, H318; Aquatic
Chronic 2, H411
01-2119543726-
33XXXX
1.0-2.0 % C14-16-18 Alkyl phenol Skin Sens. 1, H317; STOT
RE 2, H373; Aquatic
Chronic 4, H413
01-2119498288-19-
XXXX
1.0-2.0 % Reaction products of
Benzeneamine, N-phenylwith nonene (branched)
CAS:36878-20-3
EC:253-249-4
Aquatic Chronic 4, H413 01-2119488911-28-
XXXX
30.0-40.0 % Not classified oils