Several lubrication system brands in the marketplace use secondary tubing in the size of:
5/32", 4mm, 3/16", 5mm, 1/4" or 6mm. Lubecore uses 5mm OD nylon secondary tubing which in the market is sometimes referred to 3/16 because of its close proximity. However this is not to be confused with manufacturers using tubing which is exactly 3/16".
As a rule of thumb:
5mm compression fittings will compress sufficiently on 3/16" OD tubing. 3/16" OD fittings may be an extreme tight fit on 5mm tubing. Its recommended to avoid interchanging push-lock style fittings as a correct seal may not be achieved.
It is our experience that:
5/32" (3.96875mm) tube OD to 4mm compression fittings are interchangeable.
3/16" (4.7625mm) tube OD to 5mm compression fittings are interchangeable.
1/4" (6.35nn) tube OD to 6mm compression fittings are NOT interchangeable.