300tdi is RTV silicone.
As I was once told by a reputable machine shop and have since put into practise many times, any good quality silicone will do, I use unibond or another good brand and I get the interior/exterior with a 25 year lifetime.
Much cheaper than the stuff sold as RTV silicone - you get more for your money so you can give it a good coating - try not to put too much on the inside edge as you don't want it sqeezing out into the oil.
I cleaned the face of the sump and block with soapy water, then wiped it all down with alcohol, let that flash off and then applied the silicone to the sump.