I use Shell Helix 5w-40 as it's more tolerant to higher engine temperatures, normally associated with re-mapped/tuned engines. In an ideal world, you should never mix oils, but as long at you keep the first bit (5w) the same you shouldn't have any issues.
Advice: Do a good oil service next time it's due, make sure you let it drain as much as possible and clean all the filter housing thoroughly.