What would be the downside of running 0W40 Pennzoil Platinum® Euro Full Synthetic Motor Oil in a 2.1L?

I actually ran 5W40 Pennzoil Platinum® Euro Full Synthetic Motor Oil all last summer and had exceptional performance and MPG.
No oil leaks or drips, motor has 60k actual miles on it, never rebuilt.

Pennzoil Platinum® Euro is designed for complete protection and performance for European vehicles. Pennzoil Platinum Euro helps extend engine life and protects for up to 15 years or 500,000 miles, whichever comes first. Guaranteed.4 Pennzoil Platinum® Euro  is made from natural gas, not crude. Its base oil is 99.5% free from engine clogging impurities.
 
Jack R.
84 Westy with 2.1L from a 91 Syncro