40 year old motorcycle shop recommends Conventional 10-40 Castrol only

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I am reviving a 1982 Kawasaki GPz550 and asked about their recommendations for motor oil. This is a shared sump air-cooled bike.
He said they only use Castrol 10-40 conventional as they have seen too many using a synthetic have clutch slippage.

Might be right as I remember putting in, I think Motul, it is green, semi 10-40 right before I stored it and it was slipping right after the change.
I just drained the 'Motul' out (had 95 miles and 25 years on it) and used a sealed 20-year-old jug of Honda GN4 10-40 to run a bit over 1000 miles. Have a few more things to work out before I can run it down the road, so I have not been able to ride it yet.

Been pretty much a converted to full synthetic over the years so going back to a conventional is hard to get used to.

I recently bought some Mobil 1 10-40 4T bike oil and am wondering whether to heed the shops suggestion?
 
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I am reviving a 1982 Kawasaki GPz550 and asked about their recommendations for motor oil. This is a shared sump air-cooled bike.
He said they only use Castrol 10-40 conventional as they have seen too many using a synthetic have clutch slippage.

Might be right as I remember putting in, I think Motul, it is green, semi 10-40 right before I stored it and it was slipping right after the change.
I just drained the 'Motul' out (had 95 miles and 25 years on it) and used a sealed 20-year-old jug of Honda GN4 10-40 to run a bit over 1000 miles. Have a few more things to work out before I can run it down the road, so I have not been able to ride it yet.

Been pretty much a converted to full synthetic over the years so going back to a conventional is hard to get used to.

I recently bought some Mobil 1 10-40 4T bike oil and am wondering whether to head the shops suggestion?
I haven't experienced more slippage with synthetics, but I've only used synthetics stating they are good for wet clutches (Valvoline 10w40 motorcycle). With that said I do off-road only riding (ATVs and dirtbikes). I wouldn't hesitate to use any of the conventional motorcycle oils. The local bike shop uses Rotella T 15w40 in everything.
 
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It may be because it's the "grabbiest" of the oils they tried and some do slip early on but usually later on. It's probably what they've found works best for most bikes. But I wouldn't take their recommendation as gospel.
 
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KAW.jpg

I use Valvoline in mine. Works great.
Most likely those rubber connectors to the air box are hard as a rock from heat, That was the first thing I had to replace so I could clean up the carbs. They are still available from Partzilla I think.
 

cbrf3

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No airbox on this bike. The rubber boots on the engine intake were in good shape but we needed to heat them with a hair dryer for a few minutes to pop the carbs back on after they were cleaned, and all O-rings replaced. Many decades ago, I also had a 1982 KZ550 bought new, great bike!

IMG_20221118_193505538.jpg
 
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I am reviving a 1982 Kawasaki GPz550 and asked about their recommendations for motor oil. This is a shared sump air-cooled bike.
He said they only use Castrol 10-40 conventional as they have seen too many using a synthetic have clutch slippage.

Might be right as I remember putting in, I think Motul, it is green, semi 10-40 right before I stored it and it was slipping right after the change.
I just drained the 'Motul' out (had 95 miles and 25 years on it) and used a sealed 20-year-old jug of Honda GN4 10-40 to run a bit over 1000 miles. Have a few more things to work out before I can run it down the road, so I have not been able to ride it yet.

Been pretty much a converted to full synthetic over the years so going back to a conventional is hard to get used to.

I recently bought some Mobil 1 10-40 4T bike oil and am wondering whether to heed the shops suggestion?
I'm not a motorcycle guy by any means. I drove a couple of my buddies bikes back in the day. The one kid had a Kawasaki. Don't know what size it was, But I do remember it was a 4 cylinder. Was a nice riding bike with enough power for a novist like myself.
 
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Wow, I grew up on those old bikes. At 16 I took a job at Kawasaki dealership,,, just so I could ride the bikes. :) I had a 1980 KZ550 (black/chrome), a 1981 GPz550 and lastly a 1982 GPz750.

Since you already have it painted in Kawasaki's racing green colors, you should finish it off with the final touches and give it the Eddy Lawson Replica accents. Put the swingarm back to gloss black though.

And that Bassani exhaust is fantastic from that era.

Regarding the oil... API SF 10W-40 was the flavor of favor back in those wonderful days. A mineral oil designed for motorcycles will serve your purposes very well.

Let me know when you have it up and running and we can meet up for a ride up to Helen.

Kawasaki KZ1000R Eddie Lawson Replica.jpg
 
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Like @ChristianReske stated, look for the JASO T903 MA spec (or equivalent) for shared sump friction modifiers for wet clutch

Being "synthetic" has little do with clutch bite and most oils are not really a traditional synthetic anyhow
 

cbrf3

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Wow, I grew up on those old bikes. At 16 I took a job at Kawasaki dealership,,, just so I could ride the bikes. :) I had a 1980 KZ550 (black/chrome), a 1981 GPz550 and lastly a 1982 GPz750.

Since you already have it painted in Kawasaki's racing green colors, you should finish it off with the final touches and give it the Eddy Lawson Replica accents. Put the swingarm back to gloss black though.

And that Bassani exhaust is fantastic from that era.

View attachment 127093
Love the old air-cooled bikes. The bike had all of the red bits painted green before I bought it in 1997.
The pipe was near new when I got it. I plan to paint the wheels all black and the pipe needs a new coat. Bike is mostly stock and complete except the air box being gone and the Bassani Superbike pipe with a stage 3 Dynajet kit. The exhaust makes oil changes fun as the whole pipe has to be removed to replace the oil filter and pull the drain plug. I sure like the bark it has though. Just heard it for the first time in almost 25 years.
 
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The exhaust was truly a race pipe. It wasn't manufactured just for sound like they are in this day and age. The 1982 GPz was a slight detune when compared the the 1981 twin shock model - purely emissions related and brought on by a change in camshaft grind and the introduction of the CV carburetors. They responded very well to a proper 3-angle valve cut and polishing of the runners though.

I'm so glad to see one of these things being brought back to life. Bikes today are so nearly perfect that they've almost lost their souls. Good Luck with your restoration!
 
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If your motorcycle clutch slips and the plates are basket are not worn/damaged, the reason is simple, insufficient clamping force. In the case of the OP, the springs could be 40 years old, and depending on quality, they could have 2/3 (or less) of the original clamping force.

We can get wrapped around the axle about oil and oil types, but completely forget objective criteria, like measuring the actual spring force at the installed height. Every time (nearly 100% of the time) older Japanese bikes have insufficient clutch spring force, even when new. The situation does not get better as the plates wear and springs age.

That nice, easy and smooth clutch lever on Japanese motorcycles is by design.

The fix is to install stronger aftermarket springs, or shim the stock springs to the proper installed height. Done properly, the clutch will not slip with any oil choice.
 
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I had the last year Kawa 550 GPZ and that is a really nice bike. Run what ever proper rated with what ever base stock oil you want.
 
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What we call convention oil, is light years ahead and better than whatever oil they had back in the 70s or 80s. But people sure like to worry I guess.
These old timers can run pretty much anything that's a 40 weight with no worries. There was no JASO back then.

Also, agree with @Cujet about springs. Change them or at the very leas measure their length per service manual.

My 77 kz650 is doing great on Supertech diesel 15w40

BBFA5A5A-6F2F-47F9-8AF6-C6BE3BA31BBC.jpg
 
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The '82 GPz550 was THE bike back in the day. Although nothing was a scary as the H1. 😬
I owned a 1971 H1 bought new from Jack's Go Shop in San Bruno Calif. It wasn't a bit scary it was fun. The GPZ I owned the last year model wasn't as fun but is was a faster and did everything way way better and was a much better bike than the H1. The H1 is the most fun engine I have ever had. The feeling of the H1 when it hit 6,000 rpm was like getting rear ended by a semi .
 
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No airbox on this bike. The rubber boots on the engine intake were in good shape but we needed to heat them with a hair dryer for a few minutes to pop the carbs back on after they were cleaned, and all O-rings replaced. Many decades ago, I also had a 1982 KZ550 bought new, great bike!

View attachment 127089
Yeah, somebody stuck those (not so good) K&N's on mine too when I got it. They were the first things to go for my restoration. Have a newish set of 4 if you want them.
 

ZeeOSix

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I am reviving a 1982 Kawasaki GPz550 and asked about their recommendations for motor oil. This is a shared sump air-cooled bike.
He said they only use Castrol 10-40 conventional as they have seen too many using a synthetic have clutch slippage.
As others have suggested, try the Valvoline 10W-40 conventional 4-stroke motorcycle oil with the JASO MA2 rating. I've used it in many bikes (even the Hayabusa) and it works well. Walmart sells it for $6.xx per quart.

Walmart link:
 
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