Here on the neighborhood there is a workshop on ' How to make a syrup yourself '. Hiarbij is used to process 5gr levomenthol in 1 litre syrup.

Levomenthol can certainly be used for internal use because the PB V has a mint syrup, which consists of 30 g of Mint spirits and 970 g sugar syrup.  

100 g Mint Spiritus contains 1 g of mint oil and 99 g ethanol of 80 °, so 30   G contains 300 mg of mint oil

If we take the highest concentration then mint oil contains 55% menthol. Thus, 300 mg of mint oil contains 165 mg Menthol In addition to another set of other volatile flavouring substances.    

The preparations of mint syrup, which I can find on the internet, go out of fresh mint leaves. Here too, the concentration of menthol will not be very high.  

In addition, it is a small mystery to me how to dissolve 5 g levomenthol in sugar syrup.