Should you use synthetic products in place of the present lubricants? If there is a compatible product available then the answer is yes. If you have an older 4X4 then I would definitely replace the lube in the differential and transfer case the transmission is a matter of personal choice, the jury is out if synthetics will displace heat as well as transmission fluid, if you have a cooler or do not do heavy hauling then this is not a problem. I use Lube Gard,, increases the ability of transmission fluid to dissipate heat, in all my vehicles and do not use synthetic in the transmission. My 4X4 Dodge Dakota has a slight leak at the transfer case and I know synthetic would only make it worse, also I drive it only on occasion, or else I would change it. The differentials are synthetic. Synthetics will not in all cases increase a leak but it will in some cases IE the metal to metal seal on the transfer case split. First check your owner’s manual for the fluid that the manufacture recommends then go to the synthetic of choice web site and check that there a is a product available as a replacement. I binged, I use this instead of Google, and found the majority sites were Amzoil. I do not use Amzoil as it is hard to find in my area. Mobil one would be my choice. A little more money than most but they have the most experience and the largest product line. There are also some synthetic products available from the manufacturer. I would search the web for these as they are cheaper than from your local dealer. I would also check with the dealer if the vehicle is still in warranty to be sure this would not affect the it. When you first change the fluid in any gear driven part of the drive line you may at first hear a slight increase in gear noise. This is caused by a film on the gears from the old lube and this prevents the synthetic from adhering to the gear as it moves out of the fluid. This will go away in a very short period and if it does not then add a high pressure lube additive this will also help with a differential whine with regular fluid. If you rebuild a differential and replace the ring and pinion I do not recommend synthetic as a refill. The gears are self polishing from the metal particles in the lube and synthetic will not hold these in suspension. Synthetic can be installed after 3 or 4 thousand miles. Many of you who read this on a regular will be surprised by my mentioning several additives and I will explain this in greater detail at a later time.