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**BUSHWHACKER** · 15 December 2006, 15:21

Originally posted by Apache

Gonna do my oil change in a minute. I'm gonna flush with cheapo oil, then fill with decent stuff.

Am I right in thinking you cant mix synth and mineral? Soooo, am I OK flushing with cheap mineral then using a semi synth when the mineral stuff has drained?

Thinking of Magnatec for the good oil, and Halfrauds cheapo for the flush.

From the engine oil 'bible'.................

Mixing Mineral and Synthetic oils - the old and busted concepts
For the longest time, I had this to say about mixing mineral and synthetic oils:

If you've been driving around with mineral oil in your engine for years, don't switch to synthetic oil without preparation. Synthetic oils have been known to dislodge the baked-on deposits from mineral oils and leave them floating around your engine - not good. I learned this lesson the hard way! It's wise to use a flushing oil first.
If you do decide to change, only go up the scale. If you've been running around on synthetic, don't change down to a mineral-based oil - your engine might not be able to cope with the degradation in lubrication. Consequently, if you've been using mineral oil, try a semi or a full synthetic oil. By degradation, I'm speaking of the wear tolerances that an engine develops based on the oil that it's using. Thicker mineral oils mean thicker layers of oil coating the moving parts (by microns though). Switching to a thinner synthetic oil can cause piston rings to leak and in some very rare cases, piston slap or crank vibration.
Gaskets and seals! With the makeup of synthetic oils being different from mineral oils, mineral-oil-soaked gaskets and seals have been known to leak when exposed to synthetic oils. Perhaps not that common an occurrence, but worth bearing in mind nevertheless.
Mixing Mineral and Synthetic oils - the new hotness
That's the thing with progress - stuff becomes out-of-date. Fortunately for you, dear reader, the web is a great place to keep things up-to-date, so here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website.
There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synetics. Polyglycols don't mix with normal mineral oils.)
There is also no problem with switching back and forth between synthetic and mineral based oils. In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.
There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - your seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products.

Flushing oils
These are special compound oils that are very, very thin. They almost have the consistency of tap water when cold as well as hot. Typically they are 0W/20 oils. Don't ever drive with these oils in the engine - it won't last. Their purpose is for cleaning out all the gunk which builds up inside an engine. Note that Mobil1 0W40 is okay, because the '40' denotes that it's actually thick enough at temperature to work. 0W20 just doesn't get that viscous! To use them, drain your engine of all it's oil, but leave the old oil filter in place. Next fill it up with flushing oil and run it at a fast idle for about 20 minutes. Finally, drain all this off (and marvel at the $$$$ that comes out with it), replace the oil filter, refill with a good synthetic oil and voila! Clean engine.
Of course, like most things nowadays, there's a condition attached when using flushing oils. In an old engine you really don't want to remove all the deposits. Some of these deposits help seal rings, lifters and even some of the flanges between the heads, covers, pan and the block, where the gaskets are thin. I have heard of engines with over 280,000km that worked fine, but when flushed it failed in a month because the blow-by past the s$$$$er ring(now really clean)contaminated the oil and screwed the rod bearings.

Using Diesel oil for flushing
A question came up some time ago about using diesel-rated oils to flush out petrol engines. The idea was that because of the higher detergent levels in diesel engine oil, it might be a good cleaner / flusher for a non-diesel engine. Well most of the diesel oil specification oils can be used in old petrol engines for cleaning, but you want to use a low specification oil to ensure that you do not over clean your engine and lose compression for example. Generally speaking, an SAE 15W/40 diesel engine oil for about 500 miles might do the trick.
.

**Apache** · 15 December 2006, 17:24

...erm...

Drained old oil, filled with fresh 'cheapo' oil. Ran it for about 20 mins, drained oil. Just as black as the old stuff!

Removed oil filter, fitted new one, filled with GTD Magnatec. Ran engine. Much quieter! Oil pressure roughly back where I was used to it being. (a little higher perhaps, maybe due to fresh oil?)

Cheers for the tome Vince!

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