This thread makes very interesting reading and I've also know the trusty advice of the Oilman on a variety of car forums - the last one for my Fiat Coupe 20VT (via FCCUK) being the latest. (Hi Cyclone - I thought I recognised your username - I used to be JAP on FCCUK)
Anyway, I spent about 2 hours last night researching oil grading on Wiki and numerous other sites including the one owned by Oilman - Opie Oils.
I agree and understand everything written in this thread so far BUT I have two things I can't quite fathom.
1) The winter part of the grading (5
w-50) gives the cold cranking viscosity. So the lower that number the better. Am I right in thinking the cold cranking viscosity is measured on a different scale to the hot viscosity (5w-40
). If not this would imply that the oil is thinner when it is cold than when it is hot. I'm sure we all know this is not the case by general experience and also because we all warm the oil in our engine a little before draining it out - to make it thinner and gush out rather than glooping out.
2) If I'm looking for an oil with a low w number (for winter cold start protection), and a high hot temperature viscosity for protection at high running temperatures then I'm leaning towards something like a 5w-50 or possibly the 10w-60 that Alfa recommend for "decidedly spirited driving". However, on Oilmans site there is a "warning" about using oil of a 60 grade - http://www.opieoils.co.uk/pdfs/10w-60.pdf
It mentions "In a worst case scenario with thick oils (when not required) is that you will experience air entrainment and cavitation inside the bearings at high RPM."
Needless to say I don't want any air entrainment or cavitation going on in my bearings (whatever that means) so I'm thinking I should go no higher than a 50 grade.
Any ideas? At the moment I'm thinking along the lines of a good ester based fully synth with a 5w rating and a 50 hot rating - the Mobil SuperSyn 5W-50 seems about the best bet.
Any thoughts appreciated.