i have an 89 gmc r3500 that i believe has around 300000 miles but not sure because of the odometer only shows 5 digits.ive had it about 6 years now havent put alot of miles on it.should i change filter and fluid or leave it?ive read if it hasnt been change in a while and you change it it will ruin the trans.

The answer to that question, especially with your mileage, is the same as the answer to what came first, the chicken or the egg....

You will find the answer to be pro and con, almost 50%....

Me? If it ain't broke, don't fix it.....

The thought process behind the "if it ain't broke, don't fix it" is that if the transmission is about used up, and continues to work because clutch material suspended in the fluid is what is still helping to generate enough friction to make it work, when you change the fluid out, the dirty fluid full of clutch material will no longer be there, and only clean fluid will be there to make things work.

Ultimately, if the transmission is about done, fluid change could finish it off.

If the transmission is still healthy, a pan drop and fluid change should benefit it.
Also pay attention to the transmission fluid you use to fill it back up. Remember that Dexron III is no longer licensed by GM, which means there is no longer any real control over the standard. GM has approved Dexron VI for use in any of their applications that used Dexron III previously. If you use Dex/Merc to refill, there'* no telling what you're putting in.

DO NOT FLUSH the transmission, as that could break things that wouldn't otherwise break.

I agree with the others. Leave it alone.

As the old saying goes, " No such thing as a mechanic in a bottle".

I am not so sure about that, Mike.......After I have had a "few", they seem to be fixed!

ok so i wont touch it.is there any way to find out the correct miles since i can only tell it has around 80,000 miles

Run a vin check on it. Vehiclehistory.com is a free one, but experian Autocheck or CarfAx should give a history also. The history is only as good as the reports they receive however.

does that mean it wasnt reported

It'* hard to say. If it'* in excess of mechanical limits, that'* how it would show up. I usually look in the full history however. I would want to put eyes on the report (you're welcome to PM the VIN) for myself to be sure.

I tend to agree with the others and it could go either way. If it isn't showing any signs of any problems you could probably happily do a filter change, but then again why do it if it has no problems and you don't drive it much? If it is already slipping, fresh new fluid probably won't help unless you are sure the filter is plugged solid. If you flush it you're pretty much guaranteed problems. If you don't know this vehicle'* history, there'* a chance the transmission isn't

How many miles do you think is [not a lot of miles]?