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...lettters? in choosing which engine oil i should buy

 
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Real simple - chk your owner's manual or if, one is not available to you (and the oil specification is not noted on your car's oil fill cap), then contact any FRANCHISED (dealer) for your makes service department and ask. They will generally be happy to assist you.As far as the letters on varying types of oils ore concerned, they denote the specific qualities of different oils and the numbers denote the viscosity ratings.Hope this helps

 
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the letter W means weight

 
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depends on the use of your byke usually 5-15 for a 600cc or higher of road bike and thicker oil for a street bike