I would run syn lubes to be able to run longer and you will have less wear. It should shift better but that is not a given. Amsoil has a 75w-110 gear lube for the rear as well that is a good compromise between 90wt and 140wt.
A 75w-90 would be fine as well and if it was a syn would maximise drivetrain efficiency along with a syn trans fluid. The Amsoil or Redline fluids will be well worth the investment IMO.
Even though you are easy on the vehicle I would feel better using syn fluids to go 50K......
This is off the Amsoil site:
2000 FORD TRUCKS RANGER 2.5L 4-cyl Engine Code [C]
FILTERS
Oil Filter: EAO34 Absolute Efficiency Oil Filter:
Oil Filter: 51516 WIX 51516 Oil Filter
Air Filter: EAA122 Absolute Efficiency Air Filter:
Air Filter: 46253 WIX 46253 Air Filter
Fuel Filter: 33409 WIX 33409 Fuel Filter
Trans Filter: 58967 WIX 58967 Auto Tran Filter [1]
Trans Filter: 58968 WIX 58968 Auto Tran Filter [2]
1. 4R100 2WD
2. 4R100 4WD
NGK PLUGS & WIRE SETS
PLUG GPSERIES TR5GP [1]
PLUG IXIRIDIUM TR5IX NGK7397 [2]
PLUG PREMIUMPLATINUM PTR5F-11 * [3]
PLUG PRERIUMIRIDIUM ITR5F13 ^ [4]
PLUG VPOWER TR5 NGK2238 [5]
WIRESET WIRESET RC-FDZ057 NGK52090 [6]
1.Dual Plug Head
2.Dual Plug Head
3.Dual Plug Head
4.Dual Plug Head
5.Dual Plug Head
6.Dual Plug
TRICO WIPER BLADES
Blade Class Driver Passenger Rear
NEOFORM T16180 T16180
EXACTFIT T181 T181
WINTER T37180 T37180
LUBRICANTS & FLUIDS
Engine Oil
Grade 1......API*
100% Synthetic 0W-20 Motor Oil
SAE 5W-20 XL Synthetic Motor Oil
SAE 0W-30 Synthetic Signature Series Oil
All Temps......5W-20
Manual Transmission,.....MA[1]
Synthetic Automatic Transmission Fluid
Torque-Drive™ Synthetic Automatic Transmission Fluid (ATD)
Automatic Transmission,4R44E.....MA
Synthetic Automatic Transmission Fluid
Torque-Drive™ Synthetic Automatic Transmission Fluid (ATD)
Automatic Transmission,5R55E.....MA5
Synthetic Automatic Transmission Fluid
Differential, Front .....GL-5
All Temps......80W-90
SAE 80W-90 Synthetic Gear Lube
Severe Gear 75W-90
SAE 75W-90 Synthetic Gear Lube
Severe Gear 75W-110
Controlled Slip Differential, Rear .....GL-5*[2]
All Temps......80W-90
SAE 80W-90 Synthetic Gear Lube
Severe Gear 75W-90
SAE 75W-90 Synthetic Gear Lube
Severe Gear 75W-110
Standard Differential, Rear .....GL-5
All Temps......80W-90
SAE 80W-90 Synthetic Gear Lube
Severe Gear 75W-90
SAE 75W-90 Synthetic Gear Lube
Severe Gear 75W-110
Transfer Case,Warner Others.....MA
Synthetic Automatic Transmission Fluid
Torque-Drive™ Synthetic Automatic Transmission Fluid (ATD)
Transfer Case,Warner 1354.....MA
Synthetic Automatic Transmission Fluid
Torque-Drive™ Synthetic Automatic Transmission Fluid (ATD)
Fluids
Power Steering Fluid.....MA5
Synthetic Automatic Transmission Fluid
Brake Fluid.....HBH
Series 600 DOT 4 Racing Brake Fluid
Clutch Fluid.....HBH
Series 600 DOT 4 Racing Brake Fluid
1. Beginning immediately all manual transmission applications
requiring MERCON can now be serviced using MERCON-V
or MERCON Automatic Transmission Fluid or dual usage fluids
labeled MERCON/MERCON-V. After July 1, 2007,
MERCON Automatic Transmission Fluid will no longer be
manufactured, therefore, availability of this fluid will
only continue for however long it takes to deplete what
remains in inventory.
2. Limited Slip Identification: Letter & number
on plate on door lock pillar. (Models W/ Traction-Lok Rear
Axle Add 4 oz offriction modifier part no. C8AZ-19B546A
(EST-M2C118-A)
CHASSIS LUBRICATION
Two-wheel drive
0-6 Fittings, 0 Plugs..........LM
Series 2000 Synthetic Racing Grease
Synthetic GHD Heavy-Duty EP Grease
Synthetic Multi-Purpose Grease NLGI #2
Four-wheel drive
0-10 Fittings, 0 Plugs..........LM
Series 2000 Synthetic Racing Grease
Synthetic GHD Heavy-Duty EP Grease
Synthetic Multi-Purpose Grease NLGI #2
CAPACITIES
Engine, with filter..........4.6 quarts[1]
Cooling System, W/ AT Initial Fill..........10.0 quarts
AMSOIL Antifreeze and Engine Coolant
Cooling System, W/ MT Initial Fill..........11.0 quarts
AMSOIL Antifreeze and Engine Coolant
Automatic Transmission, 4R44E Initial Fill..........3.9 quarts
Automatic Transmission, 5R55E Initial Fill..........3.9 quarts
Automatic Transmission, 5R55E Initial Fill..........3.9 quarts
Automatic Transmission, Total Fill
5 speed 4wd 5R55E..........10.0 quarts
4 speed 4R44E..........9.9 quarts
5 speed 2wd 5R55E..........10.0 quarts
Manual Transmission, M50D..........5.5 pints
Differential, Front..........3.6 pints
Differential, Rear..........5.1 pints[2]
Transfer Case,Warner Others..........3.0 pints
Transfer Case,Warner 1354..........2.5 pints
1. When refilling, remove dipstick to provide adequate venting
and allow oil to flow into crankcase.
2. Fill to no more than 1/4 to 9/16 inch below fill plug hole.
TORQUES
Oil Drain Plug.....15-25 ft-lbs
Transfer case Full-Time
Fill Plug.....13 ft-lbs
Drain Plug.....13 ft-lbs
Transfer case
Fill Plug.....14-22 ft-lbs
Drain Plug.....14-22 ft-lbs
Manual Transmission Maz M50D 5-SPD
Fill Plug.....29-43 ft-lbs
Drain Plug.....29-43 ft-lbs