Any negative in using a 5w50 Synthetic oil

You will probably lose some fuel economy. A few summers ago, after I changed the oil in the tractor and the work truck at my hobby farm, I had around 3 litres of Esso XD-3 15W-40 left over. I didn't want to keep such a small amount an entire year in a 20 litre opened pail

My Prius was ready for it's 6 month oil change, so I put in the XD-3. This was in May and I ran it until Sept. No difference in how it started, or how it sounded. Compared to the Mobil 1 5W-30 I had in it, I lost a consistent 3 mpg

The used oil analysis was no better and no worse than Mobil 1. The primary advantage of XD-3 15W-40 is that if you buy a 20 litre pail, it's under $2 a litre. It's a very good HD diesel oil, completely unnecessary in a Prius

I now run Mobil 1 0W-20 year round in my Prius, with excellent used oil analsis after 12,000-16,000 km of use per oil change. If you have some 5W-50 left over, go ahead and use it up. But don't go out of your way to run it in the Prius. Most 15W-50 and 5W-50 synthetic oils have extra ZDDP additive to protect classic cars and trucks, which use solid lifters or flat tappets

Any motor with flat tappets, especially solid lifters with high spring tensions, will benefit from extra ZDDP. The additive will protect the camshaft. No need whatsoever for ZDDP in a Prius motor

The Prius in the EU allows a wide range of viscosity, including 20W-50 depending on ambient temp. Also a suggestion that 5W-30 is best, especially for cold starting. A synthetic 5W-50 will offer better cold start performance than a conventional 5W-30.

 

If ZDDP poisons catalytic converters (as I have read) than I would b enegative about oil containing it in Prius, not just neutral.

At least (as Jayman points out) higher viscosity oil will reduce fuel economy. At worst, it could delay engine starting enough to throw the code you don't want to see. Also there is at least the possibility of insufficient lubrication, because this engine is pretty tightly spec-ed. Much more comfortable considering lower viscosity engine oils in Prius.