First apologies for the long post. I have recently purchased a 97-model Mazda Xedos 6 2.0 v6 (Eunos 500 in some markets). I believe the engine is related to the Ford Duratec series of engines. The handbooks indicates that 10W30 engine oil may be used for temperatures from -20degreesC upwards, and that 5W30 may be used from 0degreesC downwards. I have some questions about this: 1) If both oil grades have 30-weight viscosity when hot, why can the 5W30 not be used all the year round? (which I later found out iswhat the previous owner did) I understand that the 5W30 will be thinner when cold - does this preclude it lubricating properly in summer temperatures during engine warm-up? 2) I have used Mobil 1 0W40 successfully in a Mazda MX5 (Miata), BMW 325i and Audi A6 with what appear to me to be good results (144, 135, 152k miles respectively) What would be the implications of using a 40-weight oil like this in the Mazda V6? - I mean in terms of long-term durability rather than rough running or noise. 3) We have a rather limited selection of oils here as compared to what appears to be the case in the USA. Of what is available, I am considering the following possibilities: 10W30: only available locally as a very cheap, non-brand-name oil. 5W30: Shell Ultra, sythetic base 0W30 or 5W30: Castrol Syntec or Magnatec 0W40: Mobil 1 I am somewhat confused as to how best to proceed, and would appreciate any advice anyone is able to give. Thanks in advance and apologies again for the long post. SEC.