Manifold temperature should never exceed 1100 degrees at manifold, or 1400 degrees at cylinder head. It start up, you need to see oil pressure come on in 6-7 minutes. A quick warm up is put brake on, rev engine a bit, and it will give you an accelerated warm up. Otherwise, 4-5 minutes at slow speeds should always be enough. Depends on ambient temperature, but could be anywhere from 4-20 minutes.
Synthetic oils are excellent, but they are very expensive.
On North Slope they recommend it for starting capabilities in extreme cold weather. Not considered suitable for most RV applications. Best to use plug-in whenever possible in cold weather. It will considerably ease starting, reducing warm up time and getting you out of the park sooner. It typically warms the engine oil to around 80 degrees, ensuring a quick start, and minimizing cranking.