I have always used oil coolers for the tranny on all my cars and never had a failure my 1986 Toyota Cressida went 529,000 no problems my 1991 Lexus LS400 315,000 and 2006 Camry 75,000 miles I use the biggest cooler I can find and use Mobile one trans. I put the trans cooler in line with my radiator on the return line going back to the trans I have driven to Death Valley many times in the summer at 129 to 134 degrees and the trans remain fairly cool. I use Toyota red antifreeze and distilled water and change all my radiator every 3 years on the button in December I use synthetic oils through out my car and never experienced any failures in any car. All the new Lexus 460 from 2007 now use all synthetic fluids through out the drive train and motor as do other high end cars today .
95-97 LS400 version had an option for tow package; includes a radiator with larger cooling capacity for coolant and ATF. If a tow package was offered with the 98 LS400, you may be able to upgrade and install all OEM parts.
using any external cooler is easy to plumb but hard to mount. the LS is designed so compact that there is hardly any room anywhere to add stuff. you will have to do some cutting to mount any external cooler in the air path.
really, i find the LS huge and quite reasonable to work on.
When I get around to ordering/mounting a trans cooler I had planned to install it behind the passenger (right) side foglights, in front of the wheel. Unlike the driver's (right) side there's no windshield washer bottle to block airflow.
1. Black on Black 1999 LS400 PM-spec
-Maya-Megan-Magnaflow-LSportline-Xenondepot-PPE Engineering-Duce-Platinum VIP-Figs Engineering-R1Concepts-
2. Black 2001 MR2 Spyder -#muchroadster-
3. Black 2004 IS300 -Manaray-HKS-Eibach-Kumho-
4. Ebony Teal 1995 LS400 11-8-06