I use synthetic oil in ALL of my "modern" cars. Without fail. Yes that is 12 quarts plus a filter for the XJS. Great stuff, no problems what so ever. Now, that being said, I would NOT RECCOMMEND synthetic oil in an old pushrod valvetrain engine. Modern oils (Synthetic and Dino) have had zinc and phospherous removed due to damage to catylitic converters. The zinc etc was there to protect the cam and lifters on "Flat tappet" engines.I spent the weekend installing an IPD cam in an old Volvo 122. Turns out IPD cams are made by Iskendarian. The cam and lifter package came with strict instructions about which oil to use to protect the cam. I have been using "Shell Rotella" in older cars because the diesel oil has higher levels of the ZDDP. I ordered some of one of the oils reccommended by Isky. I got Valvolene racing oil, (20W50)It says on the bottle "Not Street Legal" Will damage catylitic converters. The "Usuals" sell bottles of ZDDP additive to add to oil, but at the price of synthetic, it can't be cost effective to add things to it. I have no experience with the "new" Valvolene, but I use their dino and synthetic oil all the time in customer cars. Research ZDDP and draw your own conclusions though, this is just my 2 cents worth.