AirCooledSuzuki

Ok, so I’m re opening this thorny old chestnut – which is the best/most suitable oil to use in an old aircooled GSX engine, mineral oil or semi synthetic/synthetic oil?
I know this question has been asked many, many times with many and varied answers and conclusions provided by “experts” and non experts alike. Most giving apparently valid reasons for their opinion and often backed up with apparently valid technical ”evidence”.

As forums are a place to generate discussion I thought that this subject may well again generate lively debate and hopefully help me to finally make a decision on which type of oil to use in my engine.

As we all know and love these old air cooled bruisers and as I know that there are a lot of very knowledgeable people on here with huge amounts of experience with these engines in all states of tune, I thought that this would be the best place to discuss this.
For what it’s worth and if it makes any difference and as it’s me that’s looking for the advice I think it appropriate to give the specs of my engine.

OK, first up I live in Melbourne, Australia and it can get very hot here in summer, at times 40 – 44 degrees C but never gets much below 2 degrees in winter,ie it never freezes The bike will be used predominantly on the street with the occasional trip to Heathcote dragstrip.
Sooo inside we have an original gearbox, new OEM clutch friction plates with HD clutch basket, new OEM oil pump, original crank, main bearings and rods.
Again I’m not sure if this makes any difference but the engine has been “tuned”.
• MTC forged pistons and liners
• O/S S/S valves
• Hi lift cams
The original rockers and tappets are being retained as they are all in good nick and the cams aren’t too wild.

I also have top end cooling with a feed coming from the end of the oil gallery behind the cylinder block on both sides of the engine. These have dual lines coming from each side of the engine so that I have an individual feed to each cam. Also have a 13 row oil cooler fitted.

I’m not sure if these tuning mods make any difference to what oil I should use but was labouring under the assumption that tolerances back in the day of manufacture may have been a lot more generous than that used by MTC and for that matter the geezer who bored my barrels. Again labouring under the, probably mistaken, belief that with the old generous tolerances that these old motors preferred mineral oils that were, as a rule, thicker and filled up the holes and gaps that thin synthetic lubricants didn’t.

From personal experience I used to run my old tuned GSX 1170 on Castrol GTX and the like and when I changed over to semi synthetic the engine was noisier, the gearbox was notchy, the clutch slipped more easily and it just wasn’t as nice to ride.

The engine has yet to be run since it has been rebuilt by the way.

Oh and by the way I suspect that this is a prelude to the other thorny old question I suspect I'll be asking which is recomended ways of running in a new mostly new engine.

I have my own thoughts and theories on which oil to use and how to run in my engine but I'd really like to hear other peoples thoughts on these subjects as I know there are people on here that will have forgotten things that I'll probably never know.

Thanks in advance people.

1vote for mineral / semi synthetic.

Cheap and chearful 10-40 from Asda, Tesco or Morrisons..................just change it more often!

BOBSTER wrote:1vote for mineral / semi synthetic.

Cheap and chearful 10-40 from Asda, Tesco or Morrisons..................just change it more often!

Morrisons 10-40.........for me

Hi bud. Interesting thread. Not sure if u know this. 10-40 means the oil is suitable for temps of -10 degC and +40degC. Bit close for your hlt summers. I think you should aim for a 10-50.
Secondly for 20 yrs i used cheap mineral oils and changed my oil regularly. Now im older and wiser i use halfords motorcycle 10/40 oil here in the UK. Oh hang on might be 10/30 cant remember. I use motorcycle oil because its designed to cope with the clutch and fibres from it. Car engine oil doesnt need to cope with car clutches. Saying that i never had a issue for the 20 yrs i used it. The oil i use is a part sythetic. I dont like fully sythetic. It makes things rattle. Finds all the leaks etc.

Hi FBJ

Yeah I know about the winter/summer grading in the numbers. It's not a question about viscosity though. I'm interested in what peoples thoughts and experiences are with the different types of oils. Like you, I ran my GSX on mineral oil for years with no problem. Thing is though Melbourne gets a bit hotter in the summer than either Chester,Manchester or the Far North of Scotland.
Also with the extra capacity and tuning work done to the engine I'm a little concerned that the mineral oils might "break down" earlier than more modern synthetic oils.
I have heard that the molecular structure of synthetic oils are stable and consequently more effective at dispersing heat from metal components and they do so over a wider heat range. I have also heard that the molecular structure of mineral oils is unstable and that the molecules are different sizes both large and small and thus less effective at dispersing heat.

Yes also to using motorcycle only oils. This query was triggered after speaking to the Penrite Oils guy at the recent Philip Island WSB round who told me that they were soon going to re release their mineral motorcycle oil following public demand for it

I sit next to the oil specialists for our engines at work. I will ask him tomorrow and see what he thinks

Thanks mate, very much appreciated.

Just had a quick chat with our oil man.
Mineral oil. Less detergent, will coke up surfaces not excessive.
Often has a seal swell additive, normally seen on oils for older engines.
Not so good with clutches, more prone to slip.
Not to good at constant high temps. Molecules shear.

Sythetic. High detergent content. Keeps internals clean. However if your swapping from mineral to sythetic it will decoke all the deposits which will block oil ways, filters etc.
Better at high temperature. Long chain molecules don't shear.

Motorcycle oils. Much better with a wet clutch. Less prone to slip.
Better high temp stability.

Hope this helps

Hi Siminoz, If you stick with the oil that was around when the bike was built ie mineral oil I dont think you should have much bother.Like a lot of people have said change it often. One thing you dont mention is what oil pump gears you run. Use the gears from a GSX 750 et, they put MORE oil thru your engine,not high pressure, just more volume.

Hi..... theres nothing wrong with mineral oil, because that's what was around & listed for our engines back then, in these older type motors. BUT.. If you start tuning your older engines with all these performance parts, you have to move forward a bit,.. ie, you will need an oil with better shearing qualities under stress & pressure. AHEM.!!.. That's why I use Silkolene Pro4 in my Kat1428 Turbo, a synthetic fortified oil with better shearing qualities than mineral oil. That turbo impellor spins rather fast when on boost ,& it needs to cope with that extra heat that is generated also, so I don't know if normal mineral oil would hack it.
It hasn't let me down yet (touch wood!)., & the engine has remained oil tight so far.
Hope this helps.. Deanotek.