Fat is required for the absorption of carotenoids so any fat will do it . If you use very low fat dressings the absorption of these nutrients is low. This has been known for a long time.
According to one earlier trial, polyunsaturated and saturated fats raise absorption in a dose dependent fashion ie a larger amount of dressing results in a greater absorption .
The monounsaturated oil (canola/rapeseed) used in the trial seemed to have less of a dose dependent effect ie there was the same amount of carotenoid absorbed at 3g of dressing and at 20g. "Even at the lower fat level, you can absorb a significant amount of carotenoids with monounsaturated fat-rich canola oil,"
For that reason they suggest in the video on this link that Canola is a good option.
http://www.purdue.edu/newsroom/research/2012/120619FerruzziSalad.html
Olive oil is also high in monounsaturated fat and virgin olive oil retains some of the carotenoids found in the whole fruit so adding to those contained in the salad.
http://whfoods.org/genpage.php?tname=newtip&dbid=37
( the dietary 'prescription' from my dietitian, 10 years ago, included 1 tablespoon of raw olive oil on veggies or salad every day)
The fat in avocadoes eaten as fruit (rather than as oil) also increases nutrient bioavailability in the salad.
http://www.ncbi.nlm.nih.gov/pubmed/15735074
(***** the egg study was funded by the egg industry, the avocado one by the avocado growers; do a small study, get a press release and it all helps to get your product in the spotlight. )
Here's a useful chart of the different types of fats in oils
http://chartsbin.com/view/1961