whats in her now sounds like 15w30? you could go for a mono grade for summer time. eg. 30 or 40 by Redline Bursons Airport West gets it in. Otherwise use a tad of STP or Moreys oil conditioner as you need it. Hard to tell though if it only smokes after 2 hours of driving! if you say you mainly did highway driving then monograde 40 works well in the hotter months while 30 is usually good all round but not over 45 degrees ambient. Anything lower than 20w is not suitable for our climate so 20/40 is decent. You can buy 2 grades of oil and mix them in equal amounts to make your own eg. 20or30w40 that way it covers the modern engine requirements at our summer temps and back in winter go back to sae 30 etc.

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Heres a helpful chart...

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  SAEViscosity.gif (22.93 KB)