Stick to "Penetrol" paint conditioners.
They are the best. You can read about them online.
Have used for years. The best.

The newer Latex X-tender by XIM is a better bet.
You don't need very much, it's very low VOC, and doesn't affect real light colors like Floetrol (for latexes) can.
Penetrol is for oil paints, which are soon disappearing.

It's a little harder to find though...call around to your local stores.
The XIM does wonders for helping the current fast-setting latexes "level" nicely, and giving you ~ 10 minutes of working-time.
HUGE benefit here if painting doors, trims, etc.

Check out this link for the XIM info.

Faron

Here is a link that might be useful: Latex Xtender info....

doesn't affect real light colors like Floetrol (for latexes) can.

Holy cow, I am so glad to see someone else mention this. I worked with one painter who put Floetrol in e.v.e.r.y. can of paint. I think it was some sort of addiction or something. He truly believed he couldn't paint without the stuff. To say he is an eccentric gentleman is an understatement. :-D

Annnyyyywhooo. I kept telling him that Floetrol was affecting the color. I could SEE it! He said I was nuts. I thought maybe I was being too picky and let it go. Then, a little while after that TommyBoy on the PainterForum alluded to the SAME THING at one time, as in XIM is a better choice over Floetrol due to Floetrol adversely affecting color. Never have seen much about it before or since and of course when it comes to the fine nuances of color, a painter is the last person you want to ask! lol! Usually it's not their favorite *part* of the painting process to discuss at great length.