My 6hp 4 stroke mariner specifies 25w-40 for temps above 4 degrees celsius or 10w-30 for any temperature and I have been using 10w-40
Which of the above should I use?
In truth the 10-40 won't be harming it neither will a generic marine engine oil if you bought that but if you want to be spot on just buy the Quicksilver own 10-30 oil?
After all at under 0.5l for an oil change you won't be buying much anyway.
Lower number being less viscous when cold and higher number the thicker when hot, the 10/40 is more versatile however marine engine oil has a spec unused in normal motor oil that prevents foaming and breakdown after long periods of constant high revs. This is important for outboards.
Arguably more important than the grade is whether it is synthetic or not. Synthetic oil has molecules of a consistent size making it outperform regular oil and is one of the rare examples where man makes it better than nature.
The Standard Quicksilver 10w-30 is the advised oil for your OB for general use... from -18 degC to +40 degC which will pretty much cover use all around the UK/Ireland area. It doesn't state if it is synthetic or not but for this use that's less important than the fact it's what the makers advise.
