On Sunday, June 16, 2019 at 12:05:02 PM UTC-4, Frank wrote:
On 6/16/2019 8:58 AM, wrote:
Castrol extended and change oil

Synthetic oils allow for twice the mileage between oil changes. My new
car uses it and change interval in the manual is 6,000 miles or every 6
months. The Castrol is synthetic and the Pennzoil is not so you are both
right except the mechanic has shortened the synthetic period. Mechanics
and service centers are looking for business and it is best to tell them
to follow the service manual.

Doesn't sound like 2x to me. Cars were at 5K intervals with regular oil.
Even BMW went to 15K with synthetic, thought they must have seen some
problems because they backed off to 10K. If you only get 6K on synthetic,
something doesn't sound right.