Somewhere I pointed to an article where heatsink compound was compared to various alternatives.
Some of these alternatives (if I remember correctly) were toothpaste, butter, and chocolate.
In a not very surprising result, they found that almost anything worked. The reason is that the primary determinates of efficiency are:
- Being better at transferring heat than air
- Being able to form a thin layer which replaced air.
An adhesive used to mount something to a heatsink must also be able to do the above. To be effective as an adhesive, the second property is pretty much a requirement, and since air is an excellent insulator, almost anything meets the first condition above.
As in the experiment I referred to, where commercial products design specifically for the purpose were found to be better, I expect you would find the same. However, if you design conservatively, the difference may be able to be tolerated.
In the experiment I mentioned, they also commented on the fact that commercial products are designed to have long term stability in this use. They did not test, but we're reasonably confident that things like butter and toothpaste would degrade pretty quickly. In your case, adhesives are typically designed for long term use and my only concern would be the effect of maintaining long term elevated temperatures. But it may be possible to get this information from the manufacturer assuming you don't buy a no name product.