What oil to put in my Kawasaki engine?

10w-40 or 10w-50 API SG or SL and JASO MA. Rotella is cheap and meets these specs. Be sure to change on time. Some argue Syn better but again, don't stretch changes. Don't use any additives like Teflon or Slick 50.

The '305' (actually 306cc) was a bored out version of the 250cc SOHC air-cooled twin cylinder motor used in the Z250A series

It would have been fitted to the GPz305/Ninja or whatever it was called in your market. With or without belt final drive..

It uses regular 10W40 motorcycle oil.

Check the condition of the oil pump drive if you get the chance. They're made of plastic and when they fail, it destroys the top-end.

A FRANKENBIKE! Yikes! Like figure out what Kaw ("cow") used the 305 and go from there.

Use an engine oil that's specifically made for air cooled motorcycle engines. You can find it at MOST auto supply stores, or at Wal*Mart.

You can use 10W-40 API SG or 10W-40 API SL oil preferably with a JASO MA rating. Capacity is 1.8 liters.

look for some casting or parts numbers on engine case, sidecovers- then check Kawasaki dealership or web site for same numbers to get model of engine. Check for oil filter location of engine nearest to what you have, then take off cover or unscrew retainer cup to see what king of filter you have- look for part numbers on filter, cross reference if needed and get one off shelf with right number to compare. As for oil? most common Japanese engines are 'unitary' designs using same engine oil for transmission oil and have wet clutch- common default oil is 10-40 of older automotive pattern with Zinc additive or special motorcycle oil with same general additive pack-- newer auto motive 'low friction' oils have minimal or no zinc since it fouls catylitic convertors and they are the clutch foulers. Valvoline makes a good motorcycle oil in 10-40, 20-50, 15-40 and 10-30, many dealers have 'branded 'oil on shelf for their makes and models- Harley branded oil, Yamaha has 'Yamalube'. Honda has a Honda badged oil- maybe Kawasaki has a few bottles with their name on it. Many dealers have the Vavoline motorcycle oil on shelves and some have plain old formula standard oil that Honda recommended in 1070s. Diesel spec oil in 15-40 is default for many riders- delovac or rotella in gallon or 5 quart jugs for reasonable price- has the zinc for pressure film protection, higher temperature running/rating for aircooled engines, clutch safe. After finding out engine model, get a notebook for details- part numbers for oil filter, air filter, recommended spark plugs. depending on state laws- engine swaps would have inspection and source of supply sheet for engine details. And depending on the previous owner/swapper-- might be stolen engine that might have been thumped- numbers altered to match some paperwork. those jobs can be tricky to get proper parts for- like getting right starter for a Chevy pickup that came from factory with a 327 engine - but now has a Stolen 396 corvette engine in it. Need to swap nose of truck starter with onto high performance corvette patter main part. a 3 year difference in motorcycle engine production can have different heads, different motormounts on what looks like same case, 5 speed tranny instead of 4 speed. That situation is when checking sparkplugs numbers, oil filters, alternators and starter numbers on suspect engines with fudge serial numbers can find out exactly what you have.

I covered 98,000 miles on a bike once, a gpz 500, using nothing other than bog standard 10/40 oil though no doubt "experts" will warn you against this. "Proper" bike oil is a con IMO.

You can use SAE 10w-30 weight full synthetic and know that it is safe to use and will provide proper lubrication.

whats a kwasaki engine?