A have a 1 ounce bottle of poplar bud oil that is about 5 or 6+ years old. Never used any of it. Looked at it today and it is so thick (like tar) that it won't pour from the bottle. Still has the nice odor...just very thick.

Will this thin back up with some gentle heat?

Good poplar bud oil is very viscous by nature. Warming will help thin it some, however it will still be syrupy in nature if it was good material to begin with.

Thanks Bob!

Anyone willing to share what the most popular oil is to make it with?

I would thin it with mineral oil.

I like using grape seed oil.
Just make sure it's a neutral oil. No avocado or coconut oil etc...

I have made it with mineral oil before because I have this thought that food oils go rancid after time. I wanted to see if others thought there was something better. Thanks for the responses.

to prevent the oils from going rancid, keep the bottles as full as possible and tightly sealed. Oxygen is what makes oil go rancid. You can also add tocopherol (vitamin E), it is a potent antioxidant and won't change the odor of the mix.