I read an interesting article in the MGOC magazine (April 2019) by John Howard which i have thought about doing myself.
You really need to read the full article to understand the reasoning behind the mod. The standard set up sounds a very hit miss affair relying on oil being squited in the right direction to hit the top of the valve stems!!
Try and get hold of a copy of the mag or if you are a member of the MGOC you can look back at a digital copy of the mag on their website.
See my implementation of this mod at ………. https://mgb.tips/2020/09/27/extra-rocker-valve-lubrication-part-2/