Oil temperatures/weight grades?

[awa355]

Why do manufactures recommend different grades according to atmospheric tempertures? In a liquid cooled engine I would've thought the internal requirements of oil would be consistant within an engines normal operating enviroment.

At 3000 revs the properties / reqiurements of oil should be the same whether the atmosphere temp is 11c or 28c? , wouldn't it?

I can see that an air cooled engine might be more affected by outside temperture. As the oil has a bigger role as a cooling medium.

[Motu]

You have to start it at some stage,unless you want to keep it running forever.

[SMOKEU]

Put some 15/40 oil in an engine in Siberia during winter and you'll understand why.