I have a 1965 Valiant with slant six 225ci engine. It does a maximum 1000k/m a year. When it does get used most trips are more than 15 k/m and engine is always warmed up before driving off. I change the oil & filter once a year. Should I extend this change period? Oil comes out near new when drained. It uses Valvoline Xld 20w/50 sj/sl. Thanks
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