I know this item has been beaten to death in the past, but being new to the forum (and bike!) I haven't seen any real update to replacing the naff OEM view tube.
My OEM tube had a crack, resulting in a steady leak.

I opted to go with the RC tube suggestion and (after a couple of weeks of use) its holding up and operating perfectly.
The light yellow tube was 4mm internal same as stock and shows the oil level perfectly.

OEM is $20... This was $4 for two feet!!

I know right!

I added a couple of orange zip ties to either end yesterday... Sort of brings it together now.