i have alot of different oils in the 4 years ive had my 33 ( shell motul nulon royal purple redline VP racing oils and more) full synthetics are the way to go. you will also find that the older the engine the heavier wheight oil you need to use eg older 15w50 newer 5w30 or 10w40 winter you can use a slightly thinner oil it holds its consistancy in colder climate in summer the oil thins out as it gets hotter it will not protect the engine as much if you are using 5w30 or 10w40 in older engine.
the best thing to do is use an oil flush and put in a better oil.
i found that the best oils for me where royal purple, redline, VP racing and motul is borderline for me.
my 2 cents worth
cheers marcus