While preparing to change the oil on my 2002 Toyota Tundra today, I found a couple of quarts of oil in my garage that are API SL & SM. The oil I just recently bought is API SN.

I know (or at least I believe) that the newer oils are improvements over the older API designations - which makes perfect sense to me.

My question has to do with the API designation in relation to the model year of the vehicle.

In other words, API SL was good enough for my 2002 Tundra when I first bought it, so is there any reason why I shouldn’t use the older oil in my truck before getting into the newest?

Is there enough difference between API SL and API SN that would make it worth discarding the older oil and just using the new stuff?

I know I could look up Bob the Oil Guy or do some googling, but I like reading the opinions of the people here.

thanks in advance for your thoughts.

There is no reason to not use the API SL oil. No reason to discard. Use it first and if needed add either SM or SN to it to keep your oil at the right level. These oils are backwards compatible.

I have at least one vehicle that requires API SJ, and it has gotten SL and SM so far, with no negative consequences.

Happy motoring!

In general motor oils are backwards compatible and I would guess Jay is right in this matter.  However, there are exceptions.  That is one reason you will often find not one by many specs that the oil meets.  It is best to check your owner's manual.

I always thought that the newer oil designations only meant that there were more and bettter oil additives with the newer code letters. So using a quart or two of the older code letters didn’t matter much. Correst me if I am wrong.