The primary selling point of Octane is that it uses your Graphics Card (GPU) instead of your Computer Processor (CPU).

There is no "free for life" full version of Octane Render specifically for Cinema 4D R20, but there are legitimate entry points:

Would you like help finding the official demo download link or understanding GPU compatibility for your system?

Securing a professional-grade renderer for legacy versions of software like often leads users to search for "free downloads." However, it is essential to distinguish between official free versions, demo trials, and risky unauthorized downloads. Is Octane Render Free for Cinema 4D R20?

Since R20 was the first version to introduce the "ProRender" GPU engine (which never quite took off), Octane became the reliable alternative for users who wanted to use their NVIDIA cards.

If you are using the demo or have a trial/subscription, follow these steps to ensure compatibility with Release 20: Free Octane version for Cinema 4D R20? - Facebook

Strategy: Use this strictly for 14 days to learn the engine. If you have a small project due, render everything within those 14 days.