Thoughts or input on thinning 25 year old oil before an oil change.

[azmusclecar]

I am changing out all the fluids on my 79 Cutlass and I have heard, but NEVER done this, to thin out the oil with either trans fluid or
Marvel Mystery Oil or something to use or act as some sort of thinning agent and possibly cleaning agent to get as much junk
out of the block as possible. The car has been sitting for 25 years. Either that or just do an oil change and let the new oil and filter do
it's work.

I have not been running the engine until I get this settled. I am old skool and some times things of old work and some times they don't.

 

[Kennybill]

I wouldn't do it. I'd change the oil and "fill" first the new filter. Prime the engine, preferably without cranking the engine but at least without starting the engine if you do the cranking. I think I'd drive it about 500 miles then do another oil and filter change. I've seen what you mentioned done once. Engine must have had a bird inside because it started making a "chirping" noise. Hmmm, rod bearing spun, I'm sure it was just a "coincidence."

 

[ItsnotaGN]

You know, it might be worth it to pull a valve cover and see just what is lurking under there. I would be leery of using any kind of detergent additive or high detergent oil and washing any sludge into the pan where it could clog the pickup. As mentioned, if you haven't run it yet, it might be prudent to change out the oil and filter, maybe even pull the distributor and prime the pump. Sure it's a PITA, but so is rebuilding the engine. Good luck bringing it back from the dead!

 

[clean8485]

I've seen instances where someone would use 3 quarts of engine oil, and 1 quart of transmission fluid. The transmission fluid has a higher detergent content than regular engine oil. I agree with filling the filter first, and trying to prime it before starting it. I'm assuming that this car has a functioning oil pressure gauge?

 

[Gwarren79]

The way I have done it in the past was to change oil and filter, and add 1/4 cup of transmission fluid.
Change after 1 week of driving.
Repeat and then change monthly.
After 1 year the engine will be squeaky clean inside.

 

[KCP]

I always use Diesel, bio-Diesel if avail.
Out with the old oil, in with the new including a quart of Diesel, fill the filter, prime the engine for 10 minutes to flush the oil through the engine. Drop the pan and valve covers, they need to be cleaned thoroughly. Put in new oil, drive it carefully for 100mls and change oil again.

 

[azmusclecar]

Some very good ideas and suggestions. Thanks.

 

[Zeus785]

I always use Diesel, bio-Diesel if avail.
Out with the old oil, in with the new including a quart of Diesel, fill the filter, prime the engine for 10 minutes to flush the oil through the engine. Drop the pan and valve covers, they need to be cleaned thoroughly. Put in new oil, drive it carefully for 100mls and change oil again.

I have done this as well on old tractor engines to get them cleaned up. Not sure how it will work with driving it. I have always just let them run, and changed the oil every 10-15 minutes for the first hour until the oil that is changed is clean when you drain it. I also changed the filter every 2 cycles of doing this. but once again this was on old tractor engines, not sure how it will work on a car.

 

[olds307 and 403]

All good suggestions. I think the Gunk engine flush is coloured diesel. I used a few different flushes along with STP or Seafoam, ATF. I am trying to clean out my 260, poor little sludged up b*st*rd. Do an oil change and throw in some Risoline and drive, that's what I am doing.

 

[Max Headroom]

We used to do stuff like this back in the 60s. Out of maybe ten attempts by different people in different engines, I only saw it work twice. One guy put one quart of diesel in a fresh oil change in a SBC 307 with about 85,000 miles. The guy planned to drive it only to work and home for 7 days. On the third day, going to work, the oil pressure went way down and he spun two rod bearings. The second guy used a quart of engine flush in a 396/325hp. He ran the engine for about 30 minutes at an idle (per instructions) and the engine started tapping like crazy. He shut it off, pulled the valve covers off and found one of the lifters had pumped dry and collapsed. Both engines finished just exactly as dirty as they started.

If you use engine flush, Marvel Oil, diesel or anything made from lighter end hydrocarbons you run the risk of bearing contact with metal or other issues like collapsed lifters. Even transmission fluid acts as much as a pressure transmitter as a lubricant in the transmission, so its lubricating benefit to the oil is minimal. The only place an additive might give an real cleaning benefit is in those parts of the engine where that fluid is under pressure and can actually do some cleaning. However, the cleaning is as much a function of pressured flow across the dirty surface as any "cleaning value" the additive might have and those are the places that usually have the least buildup, gunk, dirt, etc.

In places where buildup usually takes place, there is not enough pressured flow to do any good. As someone mentioned above, pull a valve cover and see how much build-up you do or don't have. You will have a lot more success cleaning that area by hand than you will letting a trickle of lifter oil run over the surface of the grime.

If you have visible buildup under the valve covers, you should take the motor down and do the job right. If you just plan to drive the 305 for a few months, then leave the grime where it is until you LS. If you're not driving like a crazy person, the chances of the grime moving now after having stayed in place all this time are pretty slim.