[updraught]

It's a good question about STP as the mains are only fed by gravity. I use it in modern engines when I get an oil leak or oil usage. Works wonders. However, these mains are pressure fed.

If others have used it and the engine hasn't seized up, then that's encouraging. The product has been around a long time.

I'd only use it if I knew the mains drain tubes were clear and the internal engine clean.