It's quality oil...it's just quite thick at operating temps (obviously). Have you used similarly thick oils in that engine recently? If so, and you can't find anything else you like better, try it. I went looking for the new M1 5W-40 Turbo Diesel oil at Wal-Mart and Advanced Autoparts and they didn't have it. That should be a real winner just like M1 5W-40 T&SUV was.
I am using Syntec 5W-40 now and it seems to have sheared considerably (viscosity loss) in my 1.8T by 4000 miles into the interval. So I wouldn't recommend it for engines that prefer oils that remain 40 weights during the entire interval. It seems okay for engines that do well on thick 30 weights though. Might as well use GC over BC because they end up at similar viscosities since BC shears and GC only thickens slightly.
a friend of mine was given a 84 bmw 320 with about 220k miles on it. rusty all over but it had a new clutch and a rebuilt transmission. we took it to iffy lube and had them put in 5-30 and it clattered like it was going to come apart and the dash oil light flickered constantly. we replaced the oil sending unit and talked to a bmw mechanic. we put in 15-50 rotella diesel oil and it was so quiet you would not believe it. i will tell him about the 5-50 castrol. bmw's like a thick oil. his is supposed to run 40 at idle and 70(?) at WOT.
I just may try castrol 5w50 in my BMW. I ran 15w50 Mobil 1 during warm weather with no problems. I found that Mobil 1 0w40 just seems noisy. However, no valve clatter on first start on a cold day with 0w40.
I still get a bit of cold start/cold day clatter with 10w40 dino while doing a ARX clean/rinse so I think 10w40 is not the best.
I find it mildly creepy when people with who I'm only very vaguely familiar with know about my oil-related habits. Jr. forgot to point out that the above right bank picture shows my Audi engine at 122k miles. That picture is getting around!
Castrol 5W-50 used to be moribundman's second favorite gilded drink. ;-) It worked very well in his Audi SOHC V6 engine, but he has renounced it - another disciple of oil marketing fanaticism. Just kidding mori! ;-)
All kidding aside, C 5W-50 works very well in older Euro engines.