I have found a similar problem.    Even using Castrol 20-50, my 91 Carat will
buzz when warm.
I changed to Mobil 1 15w 50 and the light and buzzer never come on any more.
 Also available from Walmart for about $19 for 5 quarts.    I will now change
the filter and top up every 5k miles and replace oil every 15k, following
research done on the web.
I'm confident that opinions will differ but I'm converted to the synthetic
after seeing the drop in running temperature.  Temp needle stays at bottom of
LED.

cheers,
Steve Cotsford
Columbia SC