Gday all, I know this will open cans of worms and old wounds etc., but I have an oil question. My 88 k rt is about to go back out on the road. I was running gulf western 20w50 and cold start it rattled like a chaff cutter. Prior to this I ran 15w 40 but this is get harder to find for a decent price. I am considering either 10w30 or 10w 40 full synth. I am also hoping this will help with starter clutch issue which I have just pulled out and cleaned (and it needed it). The coldest it gets her is -7 Celsius (about 19.4 Fahrenheit).

Your thoughts please.

K bike are pretty bulletproof. I use vegetable oil in most of my K bikes. It's affordable and eco-friendly. I use pork lard in the older ones though.

(Disclaimer: This post may contain sarcasm.)

GeorgeDaBikie wrote:  I am considering either 10w30 or 10w 40 full synth. I am also hoping this will help with starter clutch issue which I have just pulled out and cleaned (and it needed it). The coldest it gets her is -7 Celsius (about 19.4 Fahrenheit).
 

Don't.
10W30 is really too fluid for the lubrication system, the oil pressure will drop and it won't change anything to the cold start rattle. The engine runs cold for a few seconds, but runs hot for hours! It needs a 50  viscosity grade, even a 40 will create oil spill.
Do you really ride at -7°C? Just let it warm gently for a minute before you go.
I live in the french Alps and have been riding K's daily for 30 years, summer or winter. And winters are very cold here!
20W50 il right, if you can find 15W50 or 10W50 it's OK also, as long as you stick to the 50 warm index.
Don't bother with motorcycle oils, the K100 does not require it as it has a dry clutch and runs at low RPM.
Diesel engine oil, more detergent, helps also to clean and preserve the starter clutch.
 Synth, semi-synth or mineral does not change anything either, as long as you respect the API or SAE recommended index...but overall the viscosity index!

Generally I use 20w50 dinosaur pee, but there are semi-synthetic and synthetic specialist oils available that are designed for older engines (i.e. older oil seals). Here in the UK, Millers is one such oil and I've used it without any problems. The SAE (API) index is never going to be a problem because oils have improved immensely since BMW recommended the most expensive oil-of-the-day back when the K-series was still in production. However, JBT is absolutely right when he says that you must not drop below the specified w50 heat index.

(Childish bit: I sometimes take great delight in going into the local Halfords store and looking at the car oils. When I inevitably get asked what car I need the oil for and I say it's for a motorcycle, the inevitable 'you don't want this oil sir' argument ensues. Occasionally I get someone who knows what he's talking about. Disclaimer: anyone in the UK knows that Halfords are infamous for poorly-trained technical staff, so it's a bit of a soft target really).

Dino oil: Castrol GTX 20-50 - 6K mile service interval

What I use mostly is a 50/50 mix of:
Mobil One 15-50
Mobil One High Mileage 10-40

On a 10K mile service interval.

The high mileage stuff has additives to cleanse the motor and rehab seals. My theory in using 50% HM oil is that it will help keep the starter sprag from missing and also help keep the water/oil pump and main seals from failing. Thank doG that these bikes have a dry clutch so we don't have to worry about additives.

I have no idea if this makes a material difference but so far it has worked well for me.