You can mix viscosities. It is a fairly standard trick to vary the overall viscosity. It was common practice in the 50s and 60s before multigrade oils were developed. The only issue is, the final resulting viscosity is fairly difficult to predict without properly testing it.

You will be fine to mix a synthetic 20w50 with what's in your bike to keep you going. Just change it out for something of a known quantity as soon as you get the opportunity.

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