How would I jar a sauce for keeping at room temperature without adding loads of vinegar or sugar?
You cannot use an oven for canning - while the oven itself can reach temperatures higher than 100 degrees C, it is difficult to get the internal temperature of the contents of the jars up to the same temperature. You'd be better off pressure canning, and  @Chris_H has already shown where you can read up on the technique in the comments to the question.

It might be useful to provide a quick explanation why using an oven is not reliable. Please note that I am in no way suggesting that OP's 140C is either the correct temperature or the only condition needed for safe canning.

Assuming that your contents are capable of being heated to 140C at atmospheric pressure for now, in text-book ideal conditions (assuming perfectly conductive glass), it would take 54 minutes to heat a jar of it of 10cm diameter and 10cm tall form 100C to 140C. 30 minutes is just too short.

At atmospheric pressure inside your 140C oven, your sauce will not reach too much above 100C since it is mostly made up of water. Vapourising water keeps temperature fairly constant at that, at least initially. As more and more water is evaporated, eventually, you might reach 140C, but your sauce would also be dehydrated with a very significant reduction in volume. For practical purposes, you will not see 140C for a few hours and not before your sauce is ruined. The only way to overcome that is by raising the pressure and thus the boiling point of water. This is why sterilisation needs an autoclave or at least a good pressure cooker with as high a pressure rating as possible. In any case, I have not come across a consumer type pressure cooker that would offer 3.6 bar or 52 psi to attain 140C boiling point.



edit

@rumtscho's comment below made a good point that the 140C mentioned here seemed to suggest that it was the right temperature for canning. Just to clarify, I was using OP's temperature just to illustrate the point that it would be impossible to attain that in a common oven without pressurisation. As for the need for pH control and other important conditions for safe canning, I have not attempted to address them here.