I used Castrol 20W-50 GTX for 30 years in cars and motorcycles. No problems. Three cars went over 250k miles each. Then I got a a couple of 5 qt jugs that looked like 90W. I tried it in one BMW and the Volvo and they both sounded awful on cold start. I sent a sample in to BP (owner of Castrol) and they tested it and reported that it was "within blending specifications". I asked if their specification is simply "will pour out of the jug".
Switched to Mobile 1. Problem - I cannot keep it in the engines. I have changed valve cover gaskets in both '97 328 BMWs. Changed valve cover gasket twice in the '91 245. All three still leak. One of the '97s leaks badly. Tonight the wife came in complaining that her 2004 BMW smokes and stinks. You guessed it - leaking valve cover gasket.
This is ridiculous. I have to switch to a different oil. The '97s and the '91 can go back to dino oil, bu the 2004 has to be synthetic.
What do you folks use?
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