One issue you may consider is that I believe the oil deteriorates due to contamination with old oil and purely through contact with the metal. It may be an idea to change the oil twice a year if possible, (you only need change the filter once if prefered) or perhaps use a fully synthetic or semi synthetic based oil will be better.
A lot of the people that I come into contact with say the same thing - "better to use a cheap oil and change it often than use a very expensive one and leave it in a long time."
So I guess a good compromise is to use a reasonable oil, (like a good semi-synthetic) and change it twice a year.
Just my opinion of course, as for the grade I'm not entirely sure. I bet Pete has an opinion though !!
Richard
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