The reason ford said 5W30 was to do with MPG/emissions nothing more, it's pretty universally agreed this is no good, it thins way way too much when giving the engine grief, made even worse by the very small sump capacity, oils have come a long long way since 86 when the cos came out, I would not use anything thinner than a 50 weight if a few miles on the clock, and even a 60 weight with mega high miles, I'd be tempted to use a 40 weight on a new one whilst running in.
Speaking from experience of my 200,973 MILES on original pistons bore engine, as this is what I used/did.
tabetha