I've come across joints sealed only with RTV. You have to be careful no to use too much, as it can be squeezed out into the oily side, where it can form balls. You don't want balls of RTV flowing around your oilways as it can block important bits. There are anti-ball RTVs available, although I've never tried them.

Since you already have a gasket, assuming it fits, why not use it?

The fact that, without one, you had an oil leak severe enough to fail an MOT (which I would have thought would have to be VERY severe to cause a safety issue, say by reaching the brakes, but perhaps one shouldnt try and rationalise the MOT) suggests it probably needs one.

Some sump arrangements specify both RTV and a gasket (mine requires beads of RTV in specific areas, presumably where the gap is likely to be bigger). A workshop manual should tell you whether you need both, and where to put the RTV if you do.

Failing that, a very thin coating should suffice, and will help to hold the gasket in place during assembly.

Edited by edlithgow on 04/07/2022 at 02:08