Rocker cover seal TD5

Does anyone know if you should or shouldn't use black mastic or blue hylomar seal with a rocker seal gasket on a TD5 engine. Had my lump professionally rebuilt last year but have had a persistent oil leak that I couldn't find. With the help of a phone camera I can see it is the back of the rocker cover which I am told is common for TD5s something to do with the half moon part of the gasket being difficult to get to sit properly. Unfortunately just outside my warranty on the work so need to sort it myself

You can use a little high temp stuff yes

Is it a 10P engine, if so note below caution!

The only way I've been able to get my rocker cover remotely oil tight is with sealant. I know purists would object, but it's just nicer not to live with a permanent oil slick round the engine. Plus it gets on the crank position sensor and gets inside the bellhousing and can get on the clutch. There's a small risk a dollop of sealant could block an oil drilling, but I've been OK so far.

Just clean mine and smear a little grease on it, 're used the same seal about 7 or 8 times now..