Here's AC Delco's response .. they don't even know .....
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Thank you for writing ACDelco.
With regards to your inquiry about difference between PF44 and PF46 ACDelco oil filters, we are unable to locate in our records. Please contact an ACDelco Retailer, Service Center or a GM Dealer in your area for information, as they are in the best position to assist you.
To find the nearest ACDelco Service Center in your area, please visit our website
www.acdelco.com and click on 'Service Center' under 'ACDelco Parts Retailer and Service Center Locator'. To find the nearest GM Dealership in your area, please visit the GM Goodwrench website at
www.gmgoodwrench.com, then click on 'Find a Dealer' or visit the General Motors website at
www.gm.com and click on 'Dealer Locator'.
You may also contact ACDelco Technical Customer Assistance Center at
1-800-825-5886 and select prompt #2 for information. It is a pay contact number.
With regards to your inquiry about Ultraguard Oil Filter, ACDelco manufactures only part # UPF44 and part # UPF52.
Thank you for your interest in ACDelco.
ACDelco Customer Assistance