Interesting. My 84 turbo manual doesn't mention that. In fact, 10W-30 and 5W-30 offer the same viscosity and protection once it's up to operating temperature, so there is no difference, except 5W flows better in COLD weather. I would use (and do) 5W-30 in all weather that's below about 90-100F ambient. If routinely above 100F, I'd use 10W-40.