I am going to be changing the oil on this Rover for an old friend and would like to ask everyone's opinion. Here is the skinny, '67 Land Rover 88 approximately 4000 miles on vehicle(2.25 L gas) driven less than 100 miles a year on a large hunting camp. The vehicle may sit undriven for as long as 4 months at a stretch. Typical ambient air temp while operating 10-60 degrees F. The pamplet size owner's manual recommends straight 20wt. I can get Valvoline 20wt but I was thinking that internal engine corrosion being a distinct possibility it might be wiser to go with something like a 10w30 diesel oil like Rotella T or Motorcraft both of which are easily available. Any suggestions?