I've started using synthetic oil for the 1st time in my 1997 T-bird(4.6L v8) at 80,000 miles. I've chosen Pennzoil full synthetic 10w-30. some say this is a bad oil? is this true? if it is, why is it bad? Is Castrol syntec better? doesn't it have higher ash content? Pls. help me with this. Thanks.