Dear Schraubergemeinde, here again an ÖL inquiry to the knowledgeable colleagues. For my new used BMW R50, built in 1959 I discover in the BMW original repair instructions of the Bayerische Motoren Werke AG Munich, the following specifications. Engine oil: SAE 40 transmission oil: SAE 40, from Fg.-No. 646 486 Hypoid oil SAE 90 rear wheel swing: SAE 40, from Fg.-No. 646 358 Hypoid oil SAE 90 rear wheel drive: SAE 40, from Fg.-No. 641 986 Hypoid oil SAE 90 According to the Fg.-No. of my R50, I would have to take for all SAE 40. Here my questions: What exactly is SAE 40? Is SAE 10W40 also SAE 40 or what would I have to buy? Even if I should take a test by BMW SAE 40, it would be possible or even better to take Hypoid oil SAE 90 for gearboxes, swings and end drives? Is the SAE 40 for the engine the best or if you recommend another oil?