It's very simple.
You bike is running OK on whatever oil is in it in the moment, yes? And you have no idea what specification or make of multi-grade oil is in it today, no? Good, that's that settled.... we'll come back to it later.
You can add in / top-up with just about any regularly available suitable multi-grade oil. For instance, if your bike has 20/50 in it at the moment, your could add say 20/40 which would result in a sort of 20/45 hybrid mix. Drop into your local garage, Tesco, HG or wherever and buy some. If in doubt - and to avoid too much aditional anguish - aim for something with a picture of a motorbike and 4T on the label. It will probably be synthetic or semi-synthetic; it doesn't matter which as you are not going to drink it..... Please tell me you are not planning to.
If you added 10/40 it would be a sort of 15/45 mix, depending on the ratio. For all you know it is what the last owner did and your bike is fine, is it not? See, nothing to worry about, ignorance is (in this case) bliss.
If you are still in any way nervous or unsure. Have a very quick peek in the Owner's handbook to see the 'suitable' oils BMW recommend, buy some and mix away with confidence.
PS If you are really stuck or worried. Drop me a PM. I will leave a bottle of the stuff by my front door. Have it with my compliments.
In fact that's a good idea. Ride over, heating the engine up. Put on centre stand, nice and level, outside my humble gaffe in E1. Wait four or five minutes for oil to settle. View sight screen. Top-up (if necessary) with FREE oil. Ride away.
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