One of the reasons I get my car serviced at Toyota is that when they do the service, and see something that may be a problem, they are more likely to take care of the problem then have it dwindle. If a certain part is defective, Toyota warranties cover that part and they would replace it. A quick change garage place may not know enough about the engine then just to drain oil, close drain plug, refill oil and change filter. Having said that, I am certain that most dealers would be using a low paid employee to change oil then someone who is a Master Diagnostic Technician (MDT).
Toyota's main claim to the warrantees are not to dismiss the claim, but to make sure the customer did in fact that care of the vehicle and the person who performed the service was a skilled Toyota person. Toyota (or any other dealer for that matter), would be right to say that if the wrong oil filter was used or wrong grade of oil, or if a part should have been replaced and was not, this is grounds for voiding warranty.
If you want to save the costs on the Lexus, I would suggest that you find an independent garage that has a certified or qualified mechanic for Toyota and Lexus vehicles and then get him to perform the service. In Canada, mechanics can get 30% off the parts. In that case, if you give him the jug of Mobil 1 5W30 and he supplies you the Toyota OEM oil filter and he is skilled enough to perform the necessary checks, then the odds of future problems are limited.
The thing you have to keep in mind is that if the services are performed correctly and you change your oil frequently and before the oil becomes degraded (about 5,000 KM to 6,000 KM) using a good quality motor oil with the proper grade, then I suspect that you will probably have no issues with oil gelling or sludging. Out of the 100,000's if not 1,000,000's of Toyota engines on the road, only 4,000 or so people were effected. That is about a
4% to 0.4% rate of failure which is very low.